Add gitter link to contributing

// FREEBIE

CONTRIBUTING.md

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+##Translations
+
+Please do not submit issues or pull requests for translation fixes. Anyone can update the translations in [Transifex](https://www.transifex.com/projects/p/signal-android/). Please submit your corrections there.
+
+## Submitting useful bug reports
+1. Search our issues first to make sure this is not a duplicate.
+1. Read the [Submitting useful bug reports guide](https://github.com/WhisperSystems/TextSecure/wiki/Submitting-useful-bug-reports) before posting a bug.
+
+## Development Ideology
+
+Truths which we believe to be self-evident:
+
+1. **The answer is not more options.**  If you feel compelled to add a
+   preference that's exposed to the user, it's very possible you've made
+   a wrong turn somewhere.
+1. **The user doesn't know what a key is.**  We need to minimize the points
+   at which a user is exposed to this sort of terminology as extremely as
+   possible.
+1. **There are no power users.** The idea that some users "understand"
+   concepts better than others has proven to be, for the most part, false.
+   If anything, "power users" are more dangerous than the rest, and we
+   should avoid exposing dangerous functionality to them.
+1. **If it's "like PGP," it's wrong.**  PGP is our guide for what
+   not to do.
+1. **It's an asynchronous world.**  Be wary of anything that is
+   anti-asynchronous: ACKs, protocol confirmations, or any protocol-level
+   "advisory" message.
+1. **There is no such thing as time.** Protocol ideas that require synchronized
+   clocks are doomed to failure. 
+
+Before you submit a pull request, please check if your code follows the [Code style Guidelines](https://github.com/WhisperSystems/Signal-Android/wiki/Code-Style-Guidelines).
+
+[![Gitter](https://badges.gitter.im/Join%20Chat.svg)](https://gitter.im/WhisperSystems/Signal-Android?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge)