Update the contributing guide after reliecensing (#16772)

CONTRIBUTING.md

 # Welcome to Synapse
 
-Please see the [contributors' guide](https://matrix-org.github.io/synapse/latest/development/contributing_guide.html) in our rendered documentation.
+Please see the [contributors' guide](https://element-hq.github.io/synapse/latest/development/contributing_guide.html) in our rendered documentation.

docs/development/contributing_guide.md

@@ -4,16 +4,16 @@ This document aims to get you started with contributing to Synapse!
 
 # 1. Who can contribute to Synapse?
 
-Everyone is welcome to contribute code to [Synapse](https://github.com/element-hq/synapse),
-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](https://github.com/element-hq/synapse/blob/develop/LICENSE)).
+Everyone is welcome to contribute code to
+[Synapse](https://github.com/element-hq/synapse), provided that they are willing
+to license their contributions to Element under a [Contributor License
+Agreement](https://cla-assistant.io/element-hq/synapse) (CLA). This ensures that
+their contribution will be made available under an OSI-approved open-source
+license, currently Affero General Public License v3 (AGPLv3).
 
-TODO THIS NEEDS UPDATING
+Please see the
+[Element blog post](https://element.io/blog/synapse-now-lives-at-github-com-element-hq-synapse/)
+for the full rationale.
 
 # 2. What do I need?
 
@@ -499,81 +499,19 @@ separate pull requests.)
 
 ## 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:
+After you make a PR a comment from @CLAassistant will appear asking you to sign
+the [CLA](https://cla-assistant.io/element-hq/synapse).
+This will link a page to allow you to confirm that you have read and agreed to
+the CLA by signing in with GitHub.
 
-```
-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>
-```
+Alternatively, you can sign off before opening a PR by going to
+<https://cla-assistant.io/element-hq/synapse>.
 
 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.
 
 # 10. Turn feedback into better code.