Document sections feature

[lostfictions]

Sections don't seem to be documented, though it looks like they've been supported for at least two and a half years -- I only discovered their existence by looking at the examples, and it turned out they were exactly what I needed for my particular issue.

It would be great to see the section and sections options mentioned in the readme, even if users are pointed to section-matter for the bulk of the explanation.

Relatedly, it would be nice to see #80 merged so that the feature is more usable/discoverable. (Also adding a reference to #51 here so that other folks landing on it from a search can be pointed to something a bit more recent.)