Steal contributing guidelines from Synapse

Removing all the Synapse bits

Fixes https://github.com/matrix-org/mjolnir/issues/24

CONTRIBUTING.md

@@ -0,0 +1,110 @@
+# Contributing code to Matrix
+
+Everyone is welcome to contribute code to Matrix
+(https://github.com/matrix-org), 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 our
+case, this is almost always Apache Software License v2 (see [LICENSE](LICENSE)).
+
+## How to contribute
+
+The preferred and easiest way to contribute changes to Matrix is to fork the
+relevant project on github, and then [create a pull request](
+https://help.github.com/articles/using-pull-requests/) to ask us to pull
+your changes into our repo.
+
+We use [Buildkite](https://buildkite.com/matrix-dot-org/mjolnir) for continuous
+integration. If your change breaks the build, this will be shown in GitHub, so
+please keep an eye on the pull request for feedback.
+
+To run unit tests in a local development environment, you can use `yarn test`
+and `yarn lint`.
+
+## Code style
+
+All Matrix projects have a well-defined code-style - and sometimes we've even
+got as far as documenting it... Mjolnir's code style is a relatively standard
+TypeScript project - run `yarn lint` to see how your code fairs.
+
+Before pushing new changes, ensure they don't produce linting errors.
+
+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.
+
+## 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
+[submitting patches process](
+https://www.kernel.org/doc/html/latest/process/submitting-patches.html#sign-your-work-the-developer-s-certificate-of-origin>),
+[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>
+```
+
+We accept contributions under a legally identifiable name, such as
+your name on government documentation or common-law names (names
+claimed by legitimate usage or repute). Unfortunately, we cannot
+accept anonymous contributions at this time.
+
+Git allows you to add this signoff automatically when using the `-s`
+flag to `git commit`, which uses the name and email set in your
+`user.name` and `user.email` git configs.
+
+## Conclusion
+
+That's it! Matrix is a very open and collaborative 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!