CONTRIBUTING.md proposed changes

[tenwit]

We would like to emphasize that the conventional commits are guidelines and are not intended to do be blockers. This may include rewording, and maybe reducing the "importance" of the header.

We would also like to introduce the topic of squash merges, with comments on when to consider using them and when not to bother. If this is introduced as the main topic in a section on "Commits & PRs", the conventional commits section might be made a 2nd topic in that section.

This is a follow-on from PR #14 .