Add a Gitter chat badge to README.md
Created by: gitter-badger
sourcegraph/sourcegraph now has a Chat Room on Gitter
@beyang has just created a chat room. You can visit it here: https://gitter.im/sourcegraph/sourcegraph.
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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