add some doc for docker android compilation

builds/android/README.md

+LIBZMQ ANDROID COMPILATION STEPS
+================================
+To launch the docker build:
+
+    $ docker build .
+
+To launch the android build, you have to first install the android-ndk first in your HOME directory:
+
+    $ cd ~
+    $ export ANDROID_NDK_VERSION="r10e"
+    $ wget -q http://dl.google.com/android/ndk/android-ndk-${ANDROID_NDK_VERSION}-linux-x86_64.bin -O android-ndk-${ANDROID_NDK_VERSION}-linux-x86_64.bin
+    $ chmod +x android-ndk-${ANDROID_NDK_VERSION}-linux-x86_64.bin
+    $ ./android-ndk-r10e-linux-x86_64.bin
+    $ export ANDROID_NDK_ROOT /home/zmq/android-ndk-r10e
+    $ export TOOLCHAIN_NAME arm-linux-androideabi-4.9
+    $ export TOOLCHAIN_HOST arm-linux-androideabi
+    $ export TOOLCHAIN_PATH ${ANDROID_NDK_ROOT}/toolchains/${TOOLCHAIN_NAME}/prebuilt/linux-x86_64/bin
+    $ export TOOLCHAIN_ARCH arm
+
+Then you can launch the build:
+
+    $ cd ~/libzmq/builds/android
+    $ ./build.sh 
+    Cloning into 'libsodium'...
+    remote: Counting objects: 15246, done.
+    remote: Compressing objects: 100% (182/182), done.
+    Receiving objects:  16% (2440/15246)   
+    [...]
+
+A successful build should finish with the following message and give you back your shell prompt:
+
+    [...]
+    make[2]: Leaving directory '/tmp/android_build/arm-linux-androideabi-4.9/libzmq'
+    make[1]: Leaving directory '/tmp/android_build/arm-linux-androideabi-4.9/libzmq'
+    libzmq android build succeeded
+    $ 
+
+You will then be able to see libzmq.so compiled in the prefix/ directory:
+
+    $ cd ~/libzmq/builds/android/prefix/arm-linux-androideabi-4.9/lib
+    $ ls
+    libsodium.a  libsodium.la  libsodium.so  libzmq.a  libzmq.la  libzmq.so  pkgconfig
+
+You can then triple check that they are ARM libs:
+
+    $ cd ~/libzmq/builds/android/prefix/arm-linux-androideabi-4.9/lib
+    $ file libzmq.so
+    libzmq.so: ELF 32-bit LSB shared object, ARM, EABI5 version 1 (SYSV), dynamically linked, interpreter /system/bin/linker, not stripped
+
+
+COMPILE WITH DOCKERFILE
+=======================
+
+To launch the docker build with a tagged image as a result:
+
+    $ cd ~/libzmq/builds/android
+    $ docker build -t libzmq-android:`date +"%y%m%d-%H%M%S"` .
+    $ docker build -t libzmq-android:latest .
+
+If it is successful, it will end by "libzmq android build succeeded" followed by the ContainerID "e53db616aff4":
+
+    [...]
+    make[2]: Leaving directory `/tmp/android_build/arm-linux-androideabi-4.9/libzmq'
+    make[1]: Leaving directory `/tmp/android_build/arm-linux-androideabi-4.9/libzmq'
+    libzmq android build succeeded
+    ---> e53db616aff4
+    Removing intermediate container 8a5f3e34f3da
+    Successfully built e53db616aff4
+    $
+
+    $ docker images
+    REPOSITORY          TAG                 IMAGE ID            CREATED             VIRTUAL SIZE
+    libzmq-android      151218-101411       e53db616aff4        18 hours ago        5.495 GB
+
+If you want to collect the artifacts on the build, you can specify a directory in your HOME directory, such as "$HOME/libzmq-android-bins" for example:
+    
+    $ mkdir -pv $HOME/libzmq-android-bins
+    $ docker run -v $HOME/libzmq-android-bins:/data libzmq-android:latest "cp -v /home/zmq/libzmq/builds/android/prefix/arm-linux-androideabi-4.9/lib/* /data"