Adds a GH workflow that will run `make compile-requirements` and create
a PR for it and overhauls the requirements README with info on how to
use the new workflows.
README changes:
- Group the workflow/makefile info under a new heading
- Switch to imperative for headings
- Move "Upgrade all dependencies" to end of list (uncommon for people to
actually need this command, I suspect)
- Add "Add a dependency"
- Preamble about .in and .txt files and Mac vs. Linux, as well as about forks
Also, some style fixes to upgrade-one-python-dependency GH workflow:
- Fix indentation in inputs block
- Remove "-workflow" from job name (probably copied from another workflow)
- Wrap a long line
