Add contributing guide

CONTRIBUTING.md

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+# Contributing
+
+## 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.
+
+### Private Sign off
+
+If you would like to provide your legal name privately to the Matrix.org
+Foundation (instead of in a public commit or comment), you can do so
+by emailing your legal name and a link to the pull request to
+[dco@matrix.org](mailto:dco@matrix.org?subject=Private%20sign%20off).
+It helps to include "sign off" or similar in the subject line. You will then
+be instructed further.
+
+Once private sign off is complete, doing so for future contributions will not
+be required.
+