Provide README for each module

[pombredanne]

https://github.com/nexB/purldb/tree/main/minecode and each top level module of purlDB should get it's own README file.

[AdityaSharma-Git3207]

Hi @pombredanne, I’d like to work on this issue. I’ll start by adding a README.md for minecode/ (it currently doesn’t have one), and then add READMEs for other top-level modules in separate PRs to keep the changes small and easy to review. Please let me know if this approach works.

[AdityaSharma-Git3207]

Opened PR adding minecode/README.md: #819
(refs #147).
I’ll follow up with READMEs for other top-level modules in separate small PRs to keep changes easy to review.