Add a Gitter chat badge to README.md

[gitter-badger]

ethcore/parity now has a Chat Room on Gitter

@gavofyork has just created a chat room. You can visit it here: https://gitter.im/ethcore/parity.js.

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.

[parity-cla-bot]

It looks like @gitter-badger hasn'signed our Contributor License Agreement, yet.

The purpose of a CLA is to ensure that the guardian of a project's outputs has the necessary ownership or grants of rights over all contributions to allow them to distribute under the chosen licence.
Wikipedia

You can read and sign our full Contributor License Agreement at the following URL: https://cla.ethcore.io

Once you've signed, plesae reply to this thread with [clabot:check] to prove it.

Many thanks,

Ethcore CLA Bot