Draft working conventions

Beyond our formal rules of engagement, we as a body are picking up various working conventions. These are not rules, but rather ways in which we routinely conduct our business, and it's helpful to write them down -- both as a way of inducting new people into the fold and reminding ourselves how we operate. I've tried to capture some existing conventions and introduce a few new ones in this MR.

Highlights:

• I codify our voting process for approving MRs. There is a change here, in allowing MRs to be merged before getting enough Approvals, and switching from requiring the (Vice) Secretary to merge to requiring the (Vice) Secretary to audit.
• It has come to my attention that we frequently come up with good ideas, commit to doing them, and then get distracted. This happens in private conversations and in board meetings alike. In order to hold ourselves accountable better, this MR describes a new repo internal (actually already in existence, but unused, at https://gitlab.haskell.org/hf/internal) where we would create lightweight Issues to track a task to be done. We can then perhaps make a deliberate choice to abort a task deemed unnecessary, but it seems less likely we will simply forget.
• This document defines the difference between committee (a function of the Board) and task force (a function in our executive branch).

I'm hoping this document grows over time as we establish further working conventions.

Please provide feedback! This was not drafted by a committee and so may contain all manner of errors, both in design and in execution. :)