Add contributing read me.

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Change-Id: I90b46e1907e66cde4f49eb6573beaef06c6539e9

CONTRIBUTING.md

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+# Contributing guidelines
+
+## How to become a contributor and submit your own code
+
+### Contributor License Agreements
+
+We'd love to accept your patches! Before we can take them, we have to jump a
+couple of legal hurdles.
+
+Please fill out either the individual or corporate Contributor License Agreement
+(CLA).
+
+*   If you are an individual writing original source code and you're sure you
+    own the intellectual property, then you'll need to sign an [individual
+    CLA](http://code.google.com/legal/individual-cla-v1.0.html).
+*   If you work for a company that wants to allow you to contribute your work,
+    then you'll need to sign a [corporate
+    CLA](http://code.google.com/legal/corporate-cla-v1.0.html).
+
+Follow either of the two links above to access the appropriate CLA and
+instructions for how to sign and return it. Once we receive it, we'll be able to
+accept your pull requests.
+
+***NOTE***: Only original source code from you and other people that have signed
+the CLA can be accepted into the main repository.
+
+### Contributing code
+
+If you have improvements to Sonnet, send us your pull requests! For those just
+getting started, Github has a
+[howto](https://help.github.com/articles/using-pull-requests/).
+
+If you want to contribute but you're not sure where to start, take a look at the
+[issues with the "contributions welcome"
+label](https://github.com/deepmind/sonnet/labels/stat%3Acontributions%20welcome).
+These are issues that we believe are particularly well suited for outside
+contributions, often because we probably won't get to them right now. If you
+decide to start on an issue, leave a comment so that other people know that
+you're working on it. If you want to help out, but not alone, use the issue
+comment thread to coordinate.