CONTRIBUTING: default to non-blocking comments

Simple package version updates need to include the attribute name, old and new versions, as well as a reference to the release notes or changelog.

Package upgrades with more extensive changes require more verbose commit messages.

## Merge conventions

## Review and Merge conventions

Comments on Pull Requests are considered non-blocking by default.

Every blocking comment must be explicitly marked as such by using GitHub's "Request Changes" review type.

A reviewer who submits a blocking review should be available for discussion and re-review.

An abandoned review may be dismissed after reasonable time was given at the discretion of the merger.

All suggestions for change, blocking or not, should be acknowledged before merge.

This can happen implicitly by applying the suggestion, or explicitly by rejecting it.

To make changes on commit structure and commit messages or apply simple suggestions, committers are encouraged to [checkout the PR](https://cli.github.com/manual/gh_pr_checkout) and push directly to the contributor's branch before merging.

Committers will carefully weigh the cost of another review cycle against the feelings of the contributor when pushing to their branch.