[mypyc] Small improvements to developer docs

[JukkaL]

Document tagged integers and some other updates.

[TH3CHARLie]

Looks great! Thanks for the useful updates! I am thinking how about adding a pointer to mypyc dev docs to the Quick start for contributing to mypy section in the main README.md

[JukkaL]

I am thinking how about adding a pointer to mypyc dev docs to the Quick start for contributing to mypy section in the main README.md

Yeah, we should clearly have something in there. I've been slow to do this since I've been waiting until other docs are in decent shape. I'll do another pass over the user documentation soon, and then we should be ready to have a landing page with an intro and links to docs, contributor guide and benchmark results. We also need to publish the user docs.