README and CONTRIBUTING (#3776)

... to help people who stumble on this repo.

CONTRIBUTING.md

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+Contributing to `matrix-spec-proposals`
+=======================================
+
+Thank you for taking the time to contribute to Matrix!
+
+This repository is for proposals to changes to the Matrix protocol. The process
+for submitting a proposal is spelt out in detail at
+https://spec.matrix.org/unstable/proposals (in particular,
+https://spec.matrix.org/unstable/proposals/#process).
+
+However, in brief, it amounts to:
+
+ 1. Create a [pull request][1] adding a new file to the
+    [`proposals`](./proposals) directory, expaining what you think should be
+    changed, and why. (Ideally, the file should be numbered according to the
+    pull request number, which normally means you need to rename it once you've
+    opened the PR.
+
+ 2. Seek feedback on your proposal, for example in
+    [#matrix-spec:matrix.org](https://matrix.to/#/#matrix-spec:matrix.org).
+
+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](http://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 <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 :)
+
+
+[1]: https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/proposing-changes-to-your-work-with-pull-requests/creating-a-pull-request

README.md

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+# Matrix Specification Proposals
+
+This repository contains proposals for changes to the [Matrix
+Protocol](http://spec.matrix.org), aka "Matrix Spec Changes" (MSCs).
+
+The [`proposals`](./proposals) directory contains MSCs which have been
+accepted.
+
+See [`CONTRIBUTING.md`](./CONTRIBUTING.md) for some notes on making new
+proposals. See also https://spec.matrix.org/unstable/proposals/ for more
+information on the MSC process, in particular
+https://spec.matrix.org/unstable/proposals/#process.
+
+The Matrix specification itself is maintained at
+https://github.com/matrix-org/matrix-spec.