Smaller README, improve README.rst

See https://wiki.phpmyadmin.net/pma/2014-06_Meeting#Revising_the_README_file

* Consolidate README to eliminate some duplicate information
* Editing README.rst to make it more friendly as our github home page
* Move third party license info to copyright file
Maybe it would be better to point to local file here instead of remote URL -  doc/copyright.rst