Add contribution details

The file is adopted from the Matrix project, but applies pretty well
to this SDK.

Signed-off-by: Gergely Polonkai <gergely@polonkai.eu>

CONTRIBUTING.md

@@ -0,0 +1,114 @@
+# Contributing code to the Matrix GLib SDK
+
+Everyone is welcome to contribute code to this 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 - which is the GNU Lesser
+General Public License (see COPYING).
+
+## How to contribute
+
+The preferred and easiest way to contribute changes to the Matrix GLib
+SDK is to fork the project 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.
+
+## Code style
+
+The projects have a loosely-defined code-style, which is close to
+GNOME's with a few differences. Check the source files for a hint.
+
+Please ensure your changes match the cosmetic style of the 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 this SDK is welcome to be listed in the
+AUTHORS file for the project in question. Please feel free to include a
+change to AUTHORS in your pull request to list yourself and a short
+description of the area(s) you've worked on.
+
+## 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 the SDK:
+
+    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 :)
+
+## Conclusion
+
+That's it! Similar to Matrix, this SDK is a very open and project as
+you might expect given our obsession with open communication. If we're
+going to successfully matrix together all the fragmented communication
+technologies out there we are reliant on contributions and
+collaboration from the community to do so. So please get involved -
+and we hope you have as much fun hacking on Matrix as we do!

README.md

@@ -43,6 +43,10 @@ For a working example, see [test-client.c](src/test-client.c).
 
 For a full blown example, see [test-api-client.c](src/test-api-client.c).
 
+# Contribution
+
+See the file [CONTRIBUTING.md](CONTRIBUTING.md) for details.
+
 # Code of Conduct
 
 See the file [CODE_OF_CONDUCT.md](CODE_OF_CONDUCT.md) for details.