CONTRIBUTING.md
- document `timeout` tag
- reorder tags in frontmatter doc
- minor cleanups
- minor fixes
- add style note
- reformat flags
- remove discussion of obsolete $INCLUDE
- incorporate line notes from @domenic
- integrate additional comments
- add links back, move arg notes down
- Raise outline level by one

README.md
- link to CONTRIBUTING

Minor fixes: typos, formatting, verb tense&agreement
Add example of short-list `includes: []` and long-list `includes: \n - \n -`

Add section on test environment
Add section on custom helpers
describe YAML frontmatter
Fix minor formatting errors
document $INCLUDE as obsolete
Change documentation of negative error
Move test environment and custom helpers down
indent copyright and frontmatter sections
better description of the async calls between a promise
and the functions in its `.then`
Correct Early Error example: don't throw a string

CONSOLE-RUNNER: split runner doc into new file
add troubleshooting section

add a table showing which print handle to use when
running async tests through test262.py runner

@anba contributed information about async tests in
SpiderMonkey and JavaScriptCore

add a section on requirements for using console test runner test262.py

add a section on asynchronous tests

add subsection to authoring guidelines with suggestion for test names