Add a section on defining a new event

[annevk]

Needs to cover:

• Brief description on inheritance of dictionaries and interfaces
• Using present tense for Event/type
• Event/type being all lowercase without hyphens
• Pointers to "fire", "create", and "dispatch" (and handling return values of fire and dispatch)

Anything else?

(This might involve moving some information around. I'd rather not say the same twice.)

[dbaron]

I guess the section on defining event interfaces means you've started this (although I didn't check history). (I got here from a blink-dev thread on constructors for WebAudio events.)

Possible other things:

• having a constructor
• whether to have onfoo DOM properties and (if target is an element) HTML attributes on the target
• whether to add support for document.createEvent (no, I think). (You've already mentioned not introducing init*Event, and I think this goes along.)

[domenic]

Per #372 we need to make it clear there should be a 1:1 relation between the event init dict and the event's properties