Try using cog to generate --help in README

[simonw]

Fun excuse to try cog for the first time: https://nedbatchelder.com/code/cog

[simonw]

Here are some examples of cog used in Markdown files: https://github.com/search?l=Markdown&q=cog+outl&type=Code

[simonw]

Wrote this up as a TIL: https://til.simonwillison.net/python/cog-to-update-help-in-readme