Product docs

[SmilyOrg]

Is your feature request related to a problem? Please describe.
README.md is getting out of hand. Would be nice to have actual proper "product" docs, e.g. mkdocs.

Describe the solution you'd like
https://github.com/SmilyOrg/photofield/tree/docs has a WIP of what it could look like.

Describe alternatives you've considered
Godoc, but it's very library-focused. It would be good to have docs on how to use it, explaining the different configuration options in more detail, how to install it in different ways, etc.