Define `documentation.md` practice

[jdhoffa]

          Possibly a topic for a separate discussion, but documentation should include

• Purpose of repo
• Overall structure (of repo)
• How to use (main use case, additional usecases)
  • Invoking workflow (if applicable)
  • functions of interest
• How to contribute (development usecase)

Originally posted by @AlexAxthelm in #12 (comment)