Create a CONTRIBUTING.md and document where APIs are

[Pi-Cla]

Hex has basically no documentation for their API, one has to dive into the code itself to figure out what to do across at least three different locations:

1. https://github.com/hexpm/hexpm/tree/main/lib/hexpm_web/controllers this is a place where status codes are sent
2. https://github.com/hexpm/hex/tree/main/lib/hex/api shows which URL paths and request types need to be sent
3. https://github.com/hexpm/hex/tree/main/lib/mix/tasks shows what these APIs actually do

This ticket is mostly a reminder to myself to draft a CONTRIBUTING.md as a nice simple guide to add more functions and also add comments to every single request/response pair showing where the APIs currently are for future maintenance.

Maybe if I ever feel like I have nothing better to do I will even try to document all of the things upstream, like some sorta mdbook or something which shows where all the paths are and possible status codes are... but thats for the future

[Pi-Cla]

See also my comment here: hexpm/specifications#31 (comment)

[lpil]

Good idea! Thank you