Add a Gitter chat badge to README.md

[gitter-badger]

twitter/diffy now has a Chat Room on Gitter

@travisbrown has just created a chat room. You can visit it here: https://gitter.im/twitter/diffy.

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

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

Happy chatting.

PS: Click here if you would prefer not to receive automatic pull-requests from Gitter in future.

[travisbrown]

Thanks, @gitter-badger, but we've already got one.