Add a Gitter chat badge to README.md

[gitter-badger]

recommenders/rival now has a Chat Room on Gitter

@alansaid has just created a chat room. You can visit it here: https://gitter.im/recommenders/rival.

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.

[abellogin]

I forgot about this. @alansaid, we should merge this pull, right?

[alansaid]

Well, that depends on whether we want a gitter channel for RiVal or not?