readme: add docs

Continued from #2

cc @rlidwka
Just remove the section, move it, or put it back to level 2 is all I'm saying.

Markdown spec at https://daringfireball.net/projects/markdown/basics makes it 100% clear that directly map to HTML header levels and the HTML spec http://www.w3.org/TR/html401/struct/global.html#h-7.5.5 mentions that people believe skipping is a bad practice, as it should be, since HTML does not represent styling, but document structure.