Add a CONTRIBUTING for js-sdk

... inspired by synapse's.

CONTRIBUTING.rst

@@ -1,4 +1,115 @@
 Contributing code to matrix-js-sdk
 ==================================
 
-matrix-js-sdk follows the same pattern as https://github.com/matrix-org/synapse/blob/master/CONTRIBUTING.rst
+Everyone is welcome to contribute code to matrix-js-sdk, provided that they are
+willing to license their contributions under the same license as the project
+itself. We follow a simple 'inbound=outbound' model for contributions: the act
+of submitting an 'inbound' contribution means that the contributor agrees to
+license the code under the same terms as the project's overall 'outbound'
+license - in this case, Apache Software License v2 (see `<LICENSE>`_).
+
+How to contribute
+~~~~~~~~~~~~~~~~~
+
+The preferred and easiest way to contribute changes to the project is to fork
+it on github, and then create a pull request to ask us to pull your changes
+into our repo (https://help.github.com/articles/using-pull-requests/)
+
+**The single biggest thing you need to know is: please base your changes on
+the develop branch - /not/ master.**
+
+We use the master branch to track the most recent release, so that folks who
+blindly clone the repo and automatically check out master get something that
+works. Develop is the unstable branch where all the development actually
+happens: the workflow is that contributors should fork the develop branch to
+make a 'feature' branch for a particular contribution, and then make a pull
+request to merge this back into the matrix.org 'official' develop branch. We
+use github's pull request workflow to review the contribution, and either ask
+you to make any refinements needed or merge it and make them ourselves. The
+changes will then land on master when we next do a release.
+
+We use Travis for continuous integration, and all pull requests get
+automatically tested by Travis: if your change breaks the build, then the PR
+will show that there are failed checks, so please check back after a few
+minutes.
+
+Code style
+~~~~~~~~~~
+
+The code-style for matrix-js-sdk is not formally documented, but contributors
+are encouraged to read the code style document for matrix-react-sdk
+(`<https://github.com/matrix-org/matrix-react-sdk/blob/master/code_style.md>`_)
+and follow the principles set out there.
+
+Please ensure your changes match the cosmetic style of the existing project,
+and **never** mix cosmetic and functional changes in the same commit, as it
+makes it horribly hard to review otherwise.
+
+Attribution
+~~~~~~~~~~~
+
+Everyone who contributes anything to Matrix is welcome to be listed in the
+AUTHORS.rst file for the project in question. Please feel free to include a
+change to AUTHORS.rst in your pull request to list yourself and a short
+description of the area(s) you've worked on. Also, we sometimes have swag to
+give away to contributors - if you feel that Matrix-branded apparel is missing
+from your life, please mail us your shipping address to matrix at matrix.org
+and we'll try to fix it :)
+
+Sign off
+~~~~~~~~
+
+In order to have a concrete record that your contribution is intentional
+and you agree to license it under the same terms as the project's license, we've adopted the
+same lightweight approach that the Linux Kernel
+(https://www.kernel.org/doc/Documentation/SubmittingPatches), Docker
+(https://github.com/docker/docker/blob/master/CONTRIBUTING.md), and many other
+projects use: the DCO (Developer Certificate of Origin:
+http://developercertificate.org/). This is a simple declaration that you wrote
+the contribution or otherwise have the right to contribute it to Matrix::
+
+    Developer Certificate of Origin
+    Version 1.1
+
+    Copyright (C) 2004, 2006 The Linux Foundation and its contributors.
+    660 York Street, Suite 102,
+    San Francisco, CA 94110 USA
+
+    Everyone is permitted to copy and distribute verbatim copies of this
+    license document, but changing it is not allowed.
+
+    Developer's Certificate of Origin 1.1
+
+    By making a contribution to this project, I certify that:
+
+    (a) The contribution was created in whole or in part by me and I
+        have the right to submit it under the open source license
+        indicated in the file; or
+
+    (b) The contribution is based upon previous work that, to the best
+        of my knowledge, is covered under an appropriate open source
+        license and I have the right under that license to submit that
+        work with modifications, whether created in whole or in part
+        by me, under the same open source license (unless I am
+        permitted to submit under a different license), as indicated
+        in the file; or
+
+    (c) The contribution was provided directly to me by some other
+        person who certified (a), (b) or (c) and I have not modified
+        it.
+
+    (d) I understand and agree that this project and the contribution
+        are public and that a record of the contribution (including all
+        personal information I submit with it, including my sign-off) is
+        maintained indefinitely and may be redistributed consistent with
+        this project or the open source license(s) involved.
+
+If you agree to this for your contribution, then all that's needed is to
+include the line in your commit or pull request comment::
+
+    Signed-off-by: Your Name <your@email.example.org>
+
+...using your real name; unfortunately pseudonyms and anonymous contributions
+can't be accepted. Git makes this trivial - just use the -s flag when you do
+``git commit``, having first set ``user.name`` and ``user.email`` git configs
+(which you should have done anyway :)