Add a Gitter chat badge to README.md

[gitter-badger]

Microsoft/TypeScript now has a Chat Room on Gitter

@danquirk has just created a chat room. You can visit it here: https://gitter.im/Microsoft/TypeScript.

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.

[msftclas]

Hi @gitter-badger, I'm your friendly neighborhood Microsoft Pull Request Bot (You can call me MSBOT). Thanks for your contribution!

This seems like a small (but important) contribution, so no Contribution License Agreement is required at this point. Real humans will now evaluate your PR.

TTYL, MSBOT;