Review policy for merging pull requests

The learn-ocaml project currently has 3 core co-maintainers.

The learn-ocaml review policy for merging pull requests currently amounts to the following four rules:

1. If the PR author (who can just as well be a maintainer) has a question on one's own PR
   (e.g., because there is some need for design/documentation confirmation):
   • Ask and wait for a review.
   • Merge after one approval by @erikmd or @yurug or @AltGr.
2. If no discussion looks needed, but the PR is not trivial:
   • Post a comment in the PR saying so + I'll merge the PR by This_Date (in 4 days) if no one objects by then.
   • Send an email to {EMD, YRG, LG} with the GitHub PR comment URL.
3. If the PR is trivial (e.g., for a minor bugfix or a chore commit):
   • the PR author/maintainer can merge it at once, as soon as the CI is green ✔️
4. Before any merge:
   • Assign yourself the PR to be merged.
   • Make sure the PR has a milestone.
   • Check the PR complies with best practices (atomic commits, conventional commits → CONTRIBUTING.md).
   • Check the unit- and integration-tests suite from CI is green ✔️
   • Check whether the PR must be merged as is, or squash-merged (details).
   • Merge the PR!