Document new third party self service accounts

We are moving to self service third party CI accounts. Document how that
affects third party testers in this change. Basic TL;DR is using
launchpad openid accounts so that accounts can be self managed. Then put
them in project specific voting tester groups when those accounts are
ready to vote.

Change-Id: I86edc17e13721cff0941f3286e4e10131d708735
Merge "Document new third party self service accounts"