Define the minimum number of core owner approvals

[hdgarrood]

Generally, we avoid merging things or going ahead with decisions unless at least two core owners have approved it. A point raised in discussion in #3 was that we might want to clarify this policy in this document.

@joneshf, #3 (comment)

It's been implicit for a long time that at least one other person would be involved in every decision, but never explicit that a decision might need only one person to be decided. There are surely decisions that require a quorum. And there are surely decisions that require a plenum.

@natefaubion, #3 (comment)

For this I think it would be helpful to breakdown changes in terms of:

• Maintenance (internal compiler changes)
• Compiler features (public compiler additions)
• Language additions (non-breaking additions to the surface language and semantics, but may affect tooling)
• Language changes (breaking changes to the surface language and semantics, affects the whole ecosystem)