Explain why we use a FORM here

Specifically, we use a `form` because of Dragon Dictation - users are likely to say something like "click the start button", which won't work if it's a link as Dragon doesn't obey ARIA directives right now.

Also, we use a `GET` because the downstream service subdomain `405`s `POST`s to its start page (as it should, really).

cc @Shotclog