Refactor README.md.

Move the original README.md to src since it's talking about C++
exclusively and add a more general README.md to document how
to install protoc for all languages.

src/README.md

+Protocol Buffers - Google's data interchange format
+===================================================
+
+[![Build Status](https://travis-ci.org/google/protobuf.svg?branch=master)](https://travis-ci.org/google/protobuf) [![Build status](https://ci.appveyor.com/api/projects/status/73ctee6ua4w2ruin?svg=true)](https://ci.appveyor.com/project/protobuf/protobuf)
+
+Copyright 2008 Google Inc.
+
+https://developers.google.com/protocol-buffers/
+
+C++ Installation - Unix
+-----------------------
+
+To build protobuf from source, the following tools are needed:
+
+  * autoconf
+  * automake
+  * libtool
+  * curl (used to download gmock)
+
+On Ubuntu, you can install them with:
+
+  $ sudo apt-get install autoconf automake libtool curl
+
+On other platforms, please use the corresponding package managing tool to
+install them before proceeding.
+
+If you get the source from github, you need to generate the configure script
+first:
+
+    $ ./autogen.sh
+
+This will download gmock source (which is used for C++ Protocol Buffer
+unit-tests) to the current directory and run automake, autoconf, etc.
+to generate the configure script and various template makefiles.
+
+You can skip this step if you are using a release package (which already
+contains gmock and the configure script).
+
+To build and install the C++ Protocol Buffer runtime and the Protocol
+Buffer compiler (protoc) execute the following:
+
+    $ ./configure
+    $ make
+    $ make check
+    $ sudo make install
+    $ sudo ldconfig # refresh shared library cache.
+
+If "make check" fails, you can still install, but it is likely that
+some features of this library will not work correctly on your system.
+Proceed at your own risk.
+
+For advanced usage information on configure and make, please refer to the
+autoconf documentation:
+
+  http://www.gnu.org/software/autoconf/manual/autoconf.html#Running-configure-Scripts
+
+**Hint on install location**
+
+  By default, the package will be installed to /usr/local.  However,
+  on many platforms, /usr/local/lib is not part of LD_LIBRARY_PATH.
+  You can add it, but it may be easier to just install to /usr
+  instead.  To do this, invoke configure as follows:
+
+    ./configure --prefix=/usr
+
+  If you already built the package with a different prefix, make sure
+  to run "make clean" before building again.
+
+**Compiling dependent packages**
+
+  To compile a package that uses Protocol Buffers, you need to pass
+  various flags to your compiler and linker.  As of version 2.2.0,
+  Protocol Buffers integrates with pkg-config to manage this.  If you
+  have pkg-config installed, then you can invoke it to get a list of
+  flags like so:
+
+    pkg-config --cflags protobuf         # print compiler flags
+    pkg-config --libs protobuf           # print linker flags
+    pkg-config --cflags --libs protobuf  # print both
+
+  For example:
+
+    c++ my_program.cc my_proto.pb.cc `pkg-config --cflags --libs protobuf`
+
+  Note that packages written prior to the 2.2.0 release of Protocol
+  Buffers may not yet integrate with pkg-config to get flags, and may
+  not pass the correct set of flags to correctly link against
+  libprotobuf.  If the package in question uses autoconf, you can
+  often fix the problem by invoking its configure script like:
+
+    configure CXXFLAGS="$(pkg-config --cflags protobuf)" \
+              LIBS="$(pkg-config --libs protobuf)"
+
+  This will force it to use the correct flags.
+
+  If you are writing an autoconf-based package that uses Protocol
+  Buffers, you should probably use the PKG_CHECK_MODULES macro in your
+  configure script like:
+
+    PKG_CHECK_MODULES([protobuf], [protobuf])
+
+  See the pkg-config man page for more info.
+
+  If you only want protobuf-lite, substitute "protobuf-lite" in place
+  of "protobuf" in these examples.
+
+**Note for Mac users**
+
+  For a Mac system, Unix tools are not available by default. You will first need
+  to install Xcode from the Mac AppStore and then run the following command from
+  a terminal:
+
+    $ sudo xcode-select --install
+
+  To install Unix tools, you can install "port" following the instructions at
+  https://www.macports.org . This will reside in /opt/local/bin/port for most
+  Mac installations.
+
+    $ sudo /opt/local/bin/port install autoconf automake libtool
+
+  Then follow the Unix instructions above.
+
+**Note for cross-compiling**
+
+  The makefiles normally invoke the protoc executable that they just
+  built in order to build tests.  When cross-compiling, the protoc
+  executable may not be executable on the host machine.  In this case,
+  you must build a copy of protoc for the host machine first, then use
+  the --with-protoc option to tell configure to use it instead.  For
+  example:
+
+    ./configure --with-protoc=protoc
+
+  This will use the installed protoc (found in your $PATH) instead of
+  trying to execute the one built during the build process.  You can
+  also use an executable that hasn't been installed.  For example, if
+  you built the protobuf package for your host machine in ../host,
+  you might do:
+
+    ./configure --with-protoc=../host/src/protoc
+
+  Either way, you must make sure that the protoc executable you use
+  has the same version as the protobuf source code you are trying to
+  use it with.
+
+**Note for Solaris users**
+
+  Solaris 10 x86 has a bug that will make linking fail, complaining
+  about libstdc++.la being invalid.  We have included a work-around
+  in this package.  To use the work-around, run configure as follows:
+
+    ./configure LDFLAGS=-L$PWD/src/solaris
+
+  See src/solaris/libstdc++.la for more info on this bug.
+
+**Note for HP C++ Tru64 users**
+
+  To compile invoke configure as follows:
+
+    ./configure CXXFLAGS="-O -std ansi -ieee -D__USE_STD_IOSTREAM"
+
+  Also, you will need to use gmake instead of make.
+
+**Note for AIX users**
+
+  Compile using the IBM xlC C++ compiler as follows:
+
+    ./configure CXX=xlC
+
+  Also, you will need to use GNU `make` (`gmake`) instead of AIX `make`.
+
+C++ Installation - Windows
+--------------------------
+
+If you only need the protoc binary, you can download it from the release
+page:
+
+  https://github.com/google/protobuf/releases
+
+In the downloads section, download the zip file protoc-$VERSION-win32.zip.
+It contains the protoc binary as well as public proto files of protobuf
+library.
+
+To build from source using Microsoft Visual C++, see cmake/README.md.
+
+To build from source using Cygwin or MinGW, follow the Unix installation
+instructions, above.
+
+Binary Compatibility Warning
+----------------------------
+
+Due to the nature of C++, it is unlikely that any two versions of the
+Protocol Buffers C++ runtime libraries will have compatible ABIs.
+That is, if you linked an executable against an older version of
+libprotobuf, it is unlikely to work with a newer version without
+re-compiling.  This problem, when it occurs, will normally be detected
+immediately on startup of your app.  Still, you may want to consider
+using static linkage.  You can configure this package to install
+static libraries only using:
+
+    ./configure --disable-shared
+
+Usage
+-----
+
+The complete documentation for Protocol Buffers is available via the
+web at:
+
+    https://developers.google.com/protocol-buffers/