Populate READMEs and auxiliary markdown files

[jorisdral]

The top-level README and per-package READMEs are currently empty. They should be filled in before the packages can be released.

Checklist:

• Populate README.md for fingertree-rm package. #18
• Populate README.md for diff-containers package. #19
• Populate README.md for simple-semigroupoids package. #23
• Populate top-level README.md. Update auxiliary .md files like CONTRIBUTING.md #20