The cleanup instructions previously described manually editing the `main.js` file and gave a line number that is no longer correct.
Furthermore, the `overlap` mode can be activated using a simple query parameter, no need for editing code ;)
Also simplified the markup of the contribution warning.
Using a heading is not just unnecessary as it doesn't produce any stylistic changes over bold text, but also adds a confusing heading entry to the table of contents.
- Clarify message to go to r/placeatlas2 if don't know GitHub
- Sync contents of CONTRIBUTING.md
- Make heading levels correct, make notice in html so it won't be included in GitHub TOC
- Add GitHub but commented for the time when GitHub additions are allowed
