Add a Gitter chat badge to README.md

[gitter-badger]

browserstate/history.js now has a Chat Room on Gitter

@balupton has just created a chat room. You can visit it here: https://gitter.im/browserstate/history.js.

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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