Add a Gitter chat badge to README.markdown

Created by: gitter-badger

acook/blacklight now has a Chat Room on Gitter

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

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

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

Happy chatting.

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