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 would really kill them both; they can search GitHub issues. But if I were to keep either I would keep the one at the bottom (and change the wording from "This method is obsolete" to "This method has been replaced" or something else that makes it clear it doesn't exist anymore). Putting this note in the middle kind of breaks the flow of learning with irrelevant information. *shrug*