Add a Gitter chat badge to README.md

[gitter-badger]

googlesamples/mugo now has a Chat Room on Gitter

@proppy has just created a chat room. You can visit it here: https://gitter.im/googlesamples/mugo.

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.

[googlebot]

Thanks for your pull request. It looks like this may be your first contribution to a Google open source project. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

:memo: Please visit https://cla.developers.google.com/ to sign.

Once you've signed, please reply here (e.g. I signed it!) and we'll verify. Thanks.

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[lpereira]

How can a bot sign a CLA? :)