Add a Gitter chat badge to readme.md

[gitter-badger]

aaps/stardog now has a Chat Room on Gitter

@aaps has just created a chat room. You can visit it here: https://gitter.im/aaps/stardog.

This pull-request adds this badge to your readme.md:

If my aim is a little off, please let me know.

Happy chatting.

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