Add documentation on map contributions and embedding

[candu]

Description We have a map. It's possible to contribute content to it, and to embed it elsewhere, but there's no documentation! We also haven't listed out requirements for things like logo images (size? HTTPS? format?), which might help teams.

Acceptance Criteria

• [ ] there's a file somewhere that documents these things (e.g. in a map subdirectory, or in a new guides subdirectory);
• [ ] that file is linked to from the main README.