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
Using id= breaks the internal links I added above (see current rev https://github.com/smikes/test262/commit/a112bf7b632569ec18bed26cdd7d48aeea71ab6f ) --

This link should go right to the description tag but doesn't: https://github.com/smikes/test262/blob/a112bf7b632569ec18bed26cdd7d48aeea71ab6f/CONTRIBUTING.md#tag-description