Add a Gitter chat badge to README.md

[gitter-badger]

craftyjs/Crafty now has a Chat Room on Gitter

@kevinsimper has just created a chat room. You can visit it here: https://gitter.im/craftyjs/Crafty.

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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[mucaho]

@starwed @kevinsimper
Should we give support via gitter? It's a neat platform for instant communication, plus it also archives all conversations. On the other hand we already have google group forum, should we really split up the information even more across different platforms (remember we also have to check stackoverflow for crafty related content)?