docs: add links to to README.md to home page and Zulip — heartwood

This makes it more obvious how to find more information about the project and how to talk to the project.

Signed-off-by: Lars Wirzenius liw@liw.fi

 code forges such as GitHub and GitLab that preserves user sovereignty
 and freedom.
 See the [Radicle home page](https://radicle.xyz/) for general
 information, and the [Zulip chat](https://radicle.zulipchat.com/) to
 talk to the project.
 See the [Protocol Guide](https://radicle.xyz/guides/protocol) for an
 in-depth description of how Radicle works.