Add a Gitter chat badge to README.md

[gitter-badger]

dsoprea/GDriveFS now has a Chat Room on Gitter

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

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

Happy chatting.

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[dsoprea]

Already added. Boilerplate Gitter badge is not reStructuredText friendly.