Add a Gitter chat badge to README.md

[gitter-badger]

esa/pygmo now has a Chat Room on Gitter

@krzysztof has just created a chat room. You can visit it here: https://gitter.im/esa/pygmo.

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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