document `timeout` and clean up

Fixes https://github.com/tc39/test262/issues/55

CONTRIBUTING.md
- document `timeout` tag
- reorder tags in frontmatter doc
- minor cleanups
- minor fixes
- add style note
- reformat flags
- remove discussion of obsolete $INCLUDE

README.md
- link to CONTRIBUTING
Actually, I would not include manually-created anchors at all. Even though I understand how they help in some ways, the fact that the user gets a different URL from clicking on an inter-document link vs. clicking on the link octicon that appears on hover over the relevant header is a bad tradeoff IMO.