Clarify decision making

[bryevdv]

Once https://github.com/bokeh/pm/issues/6 is accomplished, I think BEP 7: Decision Making needs to be fleshed out a bit more. It states simply:

Otherwise, a vote will be taken among all the team members.

But does not go into any actual detail about that process. E.g. it does not specify what should happen

• in the event of a tie
• in the event of abstentions (i.e. what constitutes a sufficient quorum)