Add early documentation for ART Testing support in Atest and TradeFed.

Introduce file `test/README.atest.md`, providing some initial
documentation about ART run-tests and ART gtests support in the Trade
Federation test harness, the Atest command line tool, and test
mapping.

Test: Render `README.atest.md` as HTML and check the output manually
Bug: 140373913
Bug: 147818031
Bug: 147820692
Change-Id: If1a7027a7b2cc15536b81e615118e77c5177fd80
1 file changed