PSA: refinements to contribution guidelines

Hear ye, hear ye:

Some minor refinements to the contribution guidelines were merged earlier today at https://github.com/cockroachdb/cockroach/pull/8651. Please familiarize yourself with these changes and the contribution guidelines in general.

There is also another, smaller, contribution guidelines change on-deck at https://github.com/cockroachdb/cockroach/pull/8661. I’ll let this one sit through Monday morning, in case more opinions roll in.