Clarify relationship with ATTA

[zcorpan]

From #111, @cookiecrook asked about the relationship between ARIA-AT and Assistive Technology Test Adapter. We should clarify this somewhere.

Relationship with ATTA (by Jonie, Shane).

• ATTA is an automatic test framework for OS-level accessibility API.
• ARIA-AT is for AT behavior for manual tests.

cc @halindrome

[mcking65]

We can document this in the about deck and background page.

Essentially, ATTA is for testing if a browser puts the accessibility semantics into the operating system accessibility API. ARIA-AT is for testing if the screen reader does the right things with the semantics in the API.

Said another way ... ATTA is used by ARIA WG to test browser conformance with the ARIA spec. ARIA--AT is for testing whether screen readers utilize the semantics as intended. Note, there is no such thing as conformance for AT; there is no AT spec.