docs: explain a bit about good commit messages

We didn't have any guidelines about what to include in a commit
message. Others have already written good guides for that. This commit
adds a link to one. I also added a sentence about explaining the
reason for a change, since I think that's particularly often missed
(I'm sure I also miss it sometimes - don't hesitate to point out when
that happens).

docs/contributing.md

@@ -46,7 +46,10 @@ you modified a command in the CLI, use its name as the topic, e.g
 `next/prev: <your-modification>` or `conflicts: <your-modification>`. We don't
 currently have a specific guidelines on what to write in the topic field, but
 the reviewers will help you provide a topic if you have difficulties choosing
-it.
+it. [How to Write a Git Commit Message](https://cbea.ms/git-commit/) is a good
+guide if you're new to writing good commit messages. We are not particularly
+strict about the style, but please do explain the reason for the change unless
+it's obvious.
 
 When you address comments on a PR, don't make the changes in a commit on top (as
 is typical on GitHub). Instead, please make the changes in the appropriate