Create an organisation README.

Since https://github.com/encode is our main landing page we might want to set up a README there to help direct folks around the joint?

> Add a `/profile/README.md` to the `.github` repository. The `README` will appear on your organization's profile, visible to anyone.

https://docs.github.com/en/organizations/collaborating-with-groups-in-organizations/customizing-your-organizations-profile
