Create doc/FORMS.md

[klebba]

Can be tacked into #164

[theengineear]

Note that #83 was also opened up to track this particular need. There’s more context in that ticket about some of the nuance we need to consider.

[theengineear]

Note that CustomStateSet is now baseline — this greatly improves the ability for form-associated custom elements to hook into the machinery that lands pseudo-classes on invalid form controls. I am surprised that I didn’t learn about this until just now!