Problem: No default doxygen cfg for ZeroMQ project

Solution:
- Add Doxygen.cfg file to make it simpler for library users and interested
parties to view library internals/externals
- Add README.doxygen.md as an Overview for Doxygen-generated documentation

Notes:
This could later be added into CI for autogenerated documentation that could be
posted to the gh-pages branch.

.gitignore

@@ -151,6 +151,7 @@ doc/*.3
 doc/*.7
 doc/*.html
 doc/*.xml
+doxygen/
 src/libzmq.pc
 bin/
 lib/

README.doxygen.md

+## Overview
+
+The ZeroMQ lightweight messaging kernel is a library which extends the
+standard socket interfaces with features traditionally provided by
+specialised messaging middleware products. ZeroMQ sockets provide an
+abstraction of asynchronous message queues, multiple messaging patterns,
+message filtering (subscriptions), seamless access to multiple transport
+protocols and more.
+
+This documentation describes the internal software that makes up the
+ZeroMQ C++ core engine, and not how to use its API, however it may help
+you understand certain aspects better, such as the callgraph of an API method.
+There are no instructions on using ZeroMQ within this documentation, only
+the API internals that make up the software.
+
+**Note:** this documentation is generated directly from the source code with
+Doxygen. Since this project is constantly under active development, what you
+are about to read may be out of date! If you notice any errors in the
+documentation, or the code comments, then please send a pull request.
+
+Please refer to the README file for anything else.
+## Resources
+
+Extensive documentation is provided with the distribution. Refer to
+doc/zmq.html, or "man zmq" after you have installed libzmq on your system.
+
+* Website: http://www.zeromq.org/
+* Official API documentation: http://api.zeromq.org/
+
+Development mailing list: zeromq-dev@lists.zeromq.org
+
+Announcements mailing list: zeromq-announce@lists.zeromq.org
+
+Git repository: http://github.com/zeromq/libzmq
+
+ZeroMQ developers can also be found on the IRC channel \#zeromq, on the
+Freenode network (irc.freenode.net).
+
+## Copyright
+Copyright (c) 2007-2016 Contributors as noted in the AUTHORS file.  
+The project license is specified in COPYING and COPYING.LESSER.
+
+The names "ØMQ", "ZeroMQ", "0MQ", and the ØMQ logo are registered trademarks
+of iMatix Corporation ("iMatix") and refers to either (a) the original libzmq
+C++ library, or (b) the community of projects hosted in the
+https://github.com/zeromq organization.
+
+This Doxygen configuration is adapted by Hiten Pandya, for the ZeroMQ project.