Add documentation about how we handle incoming breakages

[guarani]

If a release is cut and an issue is found, we might first classify the issue as a blocker or non-blocker and then we must decide whether to continue the release, revert the offending commit (if we've located it e.g. via git bisect), or pause the release while a fix is found.

It would be useful to capture these three scenarios in the documentation so that we have some established guidelines for how to proceed.