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
I wanted to leave some breadcrumbs in case someone who used to write test262 tests comes back and wonders "What happened to $INCLUDE?"  I axed the section at the end that used to say "$INCLUDE is obsolete."  Which do you think would be better to keep, or really kill them both?