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.
I'm using capitalization to refer to the sections (which are also captialized), but I should probably put a link around those.

Now to the issue at hand: What I am trying to say is that the spec is not something stable and we may always change the Parser section. Therefore, not all implemented parsers may adhere to the same rules at the same time and old implementations may not yet have caught up with new rules. This is how divergence can happen.

Do you have a suggestion for how to improve the wording?