github: Add README.md

This file will be rendered if the repository is accessed via the GitHub
website. It contains the most imporant information from `/README.md`
and some additional information regarding the status of the repository
on GitHub.

Thanks for the pointer, Yorgos!

See also:
 - https://radicle.zulipchat.com/#narrow/channel/369277-heartwood/topic/github.20repo/with/522296878
 - https://docs.github.com/en/repositories/managing-your-repositorys-settings-and-features/customizing-your-repository/about-readmes

Co-authored-by: Fintan Halpenny <fintan.halpenny@gmail.com>

.github/README.md

@@ -0,0 +1,56 @@
+# ❤️🪵
+
+*Radicle Heartwood Protocol & Stack*
+
+Heartwood is the third iteration of the Radicle Protocol, a powerful
+peer-to-peer code collaboration and publishing stack. The repository contains a
+full implementation of Heartwood, complete with a user-friendly command-line
+interface (`rad`) and network daemon (`radicle-node`).
+
+[Radicle][homepage] is a secure, decentralized and powerful alternative to
+centralized code forges such as GitHub and GitLab that preserves user
+sovereignty and freedom.
+
+See the [Protocol Guide][guides-protocol] for an in-depth description of how
+Radicle works.
+
+For more information about [installing][install] and [running][run] Radicle, how
+to contribute, and licensing, please refer to the [main `README.md` of the
+repository][readme].
+
+## Radicle on GitHub
+
+Please note that GitHub is not the forge that Radicle is being developed with.
+Radicle is developed using Radicle. Therefore, issues cannot be created on
+GitHub, but only via Radicle directly. However, pull requests are still
+accepted.
+
+To view existing issues and patches with your web browser, navigate to
+[app.radicle.xyz][heartwood].
+
+To contribute issues and patches, please consider [installing][install]
+Radicle and follow our [guides][guides].
+
+## Feedback
+
+If you have feedback, feel free to create issues using `rad issue`, join
+[our Zulip][zulip], or email [feedback@radicle.xyz][mail-feedback].
+Emails sent to this address are [automatically posted][zulip-help-email] to
+[our **public** #feedback channel on Zulip][zulip-feedback], revealing the
+[`From` header][rfc2822s3.6.2] (which usually contains your name and email
+address). This allows us to discuss your feedback on Zulip, and, if necessary,
+respond to you via email.
+
+
+[guides-protocol]: https://radicle.xyz/guides/protocol
+[guides]: https://radicle.xyz/guides
+[heartwood]: https://app.radicle.xyz/nodes/seed.radicle.xyz/rad:z3gqcJUoA1n9HaHKufZs5FCSGazv5
+[homepage]: https://radicle.xyz
+[install]: ../README.md#installation
+[mail-feedback]: mailto:feedback@radicle.xyz
+[readme]: ../README.md
+[rfc2822s3.6.2]: https://datatracker.ietf.org/doc/html/rfc2822#section-3.6.2
+[run]: ../README.md#running
+[zulip-feedback]: https://radicle.zulipchat.com/#narrow/channel/392584-feedback
+[zulip-help-email]: https://talently.zulip.com/help/message-a-channel-by-email
+[zulip]: https://radicle.zulipchat.com/