Document implementation-contributed

[dminor]

This is based upon the staging section. Unlike the previous implementation-contributed, this one is a subdirectory under tests.

I've left the requirement for the tests to be runnable under the usual test262 runner. This isn't a problem for SpiderMonkey, since we have a conversion script, but we could drop it if we think this is too much of a burden for other implementations. Personally, I think implementation-contributed will be more useful if we keep it.