Add a conformance section.

Closes https://www.w3.org/Bugs/Public/show_bug.cgi?id=22591

Let me know if you think this is sufficient.
This makes it sounds like it's acceptable for a parser to diverge from the spec, which it of course is not. It's only if all (relevant) parsers are per spec that we will have the ability to use the extensibility built into it. (Nit: Parsers shouldn't be capitalized.)