Generate README.md from sudo-edit.el using make-readme-markdown

[mgalgs]

This adds support for generating our README.md from the sudo-edit.el source itself using make-readme-markdown [1].

This is nice because, as per the MRM docs [1], "when you're writing an elisp module, the module itself should be the canonical source of documentation. But it's not very user-friendly or good marketing for your project to have an empty README.md that refers people to your source code, and it's even worse if you have to maintain two separate files that say the same thing."

[1] https://github.com/mgalgs/make-readme-markdown

[mgalgs]

See my fork for a preview of the rendered markdown looks: https://github.com/mgalgs/sudo-edit