Add a Gitter chat link to README

[gitter-badger]

elacin/PDFExtract now has a Chat Room on Gitter

@elacin has just created a chat room. You can visit it here: https://gitter.im/elacin/PDFExtract.

This pull-request adds a link to your README.

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

Happy chatting.

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