Add a Gitter chat link to README.txt

[gitter-badger]

antlr/antlr3 now has a Chat Room on Gitter

@sharwell has just created a chat room. You can visit it here: https://gitter.im/antlr/antlr3.

This pull-request adds a link to your README.txt.

If my aim is a little off, please let me know.

Happy chatting.

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