Add TOC to README.md

[benedeki]

Background

README.md getting long and harder to orient in.

Feature

Add a TOC into the file.

[lsulak]

Perhaps this is no longer needed since Github added a new feature ~1 year ago: https://github.blog/changelog/2021-04-13-table-of-contents-support-in-markdown-files/

This is how it looks like: