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
Is this still true? Even for @bterlson's Node runner?