Documentation request: describe and link to documentation for the tech stack + related CRHQ projects

[EMCain]

It would be good to write a section in the README listing the various packages and technologies this relies on, to the extent that people need to understand them, with a short description of how each is used, and links to documentation for them. Also, describing any other CRHQ projects it interacts with, and external APIs.

I can work on writing this, but I will need help with some parts.

[ArtOfCode-]

All yours, Emily :) Ping the usual suspects in Slack if you need info; the thing currently on the top of my head is that it will help to have a good understanding of MySQL and RDB concepts in general.

[thesecretmaster]

Also, feel free to make a PR. If you do, I can help write bits.