CONTRIBUTING: add notes on async

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
Merge pull request #53 from smikes/CONTRIBUTING-async-printhandles

CONTRIBUTING: add notes on async
Fix suspect formatting in S15.2.3.1_A3
CONTRIBUTING: more notes on test writing