Add a CONTRIBUTING file

CONTRIBUTING.rst

+Contributing code to libolm
+===========================
+
+To contribute code to this library, the preferred way is to clone the git
+repository, create a git patch series (for example via ``git
+format-patch --stdout origin/master``), and send this by email to
+``richard@matrix.org``.
+
+Naturally, you must be willing to license your contributions under the same
+license as the project itself - in this case, Apache Software License v2 (see
+`<LICENSE>`_).
+
+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 commits or covering email::
+
+    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 :)

README.rst

@@ -129,6 +129,10 @@ Olm uses pure C implementations of the cryptographic primitives used by
 the ratchet. While this decreases the performance it makes it much easier
 to compile the library for different architectures.
 
+Contributing
+------------
+Please see `<CONTRIBUTING.rst>`_ when making contributions to the library.
+
 What's an olm?
 --------------