README: format in Markdown?

[mxlje]

Is there a specific reason that the README is a plaintext file instead of Markdown? Markdown formatting would make it easier to see the different sections and we could even add a TOC. Additionally, it would prevent horizontal scrolling on the GitHub mobile site, making the document a lot easier to read.

I would be happy to open a PR for this, just want to make sure I’m not missing anything obvious.

Thank you 🎉