Make CONTRIBUTING.md consistent with TensorFlow (#580)

Simplify CONTRIBUTING.md by copying the prose from TensorFlow.

The current text is a bit chatty, and since we aren't lawyers I think we shouldn't go too much into explaining the legal requirements. Also added a mention that we can help people write plugins.

Author: Dandelion Mané

CONTRIBUTING.md

@@ -1,26 +1,22 @@
-Want to contribute? Great! First, read this page (including the small print at the end).
-
-### Before you contribute
-Before we can use your code, you must sign the
-[Google Individual Contributor License Agreement]
-(https://cla.developers.google.com/about/google-individual)
-(CLA), which you can do online. The CLA is necessary mainly because you own the
-copyright to your changes, even after your contribution becomes part of our
-codebase, so we need your permission to use and distribute your code. We also
-need to be sure of various other things—for instance that you'll tell us if you
-know that your code infringes on other people's patents. You don't have to sign
-the CLA until after you've submitted your code for review and a member has
-approved it, but you must do so before we can put your code into our codebase.
-Before you start working on a larger contribution, you should get in touch with
-us first through the issue tracker with your idea so that we can help out and
-possibly guide you. Coordinating up front makes it much easier to avoid
-frustration later on.
+### 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.
+
+
+### Working with the team
+If you're planning a larger contribution, please get in touch with the team through a GitHub issue before starting work - we can help guide you, and coordinating up front will make the process smoother.
+
+If you want to add a major feature, it may be a good candidate for adding a plugin. Let us know via a GitHub issue, and we can guide you in the process.
 
 ### Code reviews
 All submissions, including submissions by project members, require review. We
 use GitHub pull requests for this purpose.
-
-### The small print
-Contributions made by corporations are covered by a different agreement than
-the one above, the
-[Software Grant and Corporate Contributor License Agreement](https://cla.developers.google.com/about/google-corporate).