Moved to a more comprehensive contributing doc.

Based on the How To Contribute wiki page (and switched to markdown so
it's a simple copy-paste).

CONTRIBUTING.md

+Contributions are very welcome. The easiest way is to fork the repo and then make a pull request from your fork. Before your pull request is merged, it will be reviewed by at least one person. There may be feedback so expect comments on the pull request. Add yourself to the AUTHORS file in your first pull request.
+
+Please review:
+
+* [[Python Guidelines]]
+* [[Javascript Guidelines]]
+* [Testing](https://github.com/edx/edx-platform/blob/master/docs/internal/testing.md)
+
+Coding conventions should be followed and your commit should *increase* test coverage, not decrease it. For more involved contributions, you may want to discuss your intentions on the mailing list *before* you start coding.
+
+Before your first pull request is merged, you'll need to sign the [individual contributor agreement](http://code.edx.org/individual-contributor-agreement.pdf) and send it in. This confirms you have the authority to contribute the code in the pull request and ensures we can relicense it.
+
+If you have any questions, please ask on the [mailing list](https://groups.google.com/forum/#!forum/edx-code).

CONTRIBUTING.rst

-How To Contribute
-=================
-
-Contributions are very welcome.
-
-Please read `How To Contribute <https://github.com/edx/edx-platform/wiki/How-To-Contribute>`_ for details.