add DCO (element-hq#3395)

The Matrix.org Foundation is rolling out this DCO across all public
repositories in its namespace. Of note, this DCO does not require the
use of "real" or "legally identifiable" names.

### Pull Request Checklist

<!-- Please read
https://matrix-org.github.io/dendrite/development/contributing before
submitting your pull request -->

* [X] I have added Go unit tests or [Complement integration
tests](https://github.com/matrix-org/complement) for this PR _or_ I have
justified why this PR doesn't need tests
* [X] Pull request includes a [sign off below using a legally
identifiable
name](https://matrix-org.github.io/dendrite/development/contributing#sign-off)
_or_ I have already signed off privately

Signed-off-by: `Josh Simmons <[email protected]>`

Co-authored-by: Josh Simmons <[email protected]>

CONTRIBUTING.md

@@ -0,0 +1,59 @@
+# Contributing to Dendrite
+
+Thank you for taking the time to contribute to Matrix!
+
+This is the repository for Dendrite, a second-generation Matrix homeserver written in Go.
+
+## Sign off
+
+We ask that everybody who contributes to this project signs off their contributions, as explained below.
+
+We follow a simple 'inbound=outbound' model for contributions: the act of submitting an 'inbound' contribution means that the contributor agrees to license their contribution under the same terms as the project's overall 'outbound' license - in our case, this is Apache Software License v2 (see [LICENSE](./LICENSE)).
+
+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 used by the [Linux Kernel](https://www.kernel.org/doc/html/latest/process/submitting-patches.html), [Docker](https://github.com/docker/docker/blob/master/CONTRIBUTING.md), and many other projects: the [Developer Certificate of Origin](https://developercertificate.org/) (DCO). 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 <[email protected]>
+```
+
+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.