CONTRIBUTING: add documentation of our commit message style

While we try to conform to a certain commit message style, this style
has never been documented anywhere. Document it such that new
contributors do not have to go through another needless iteration of
their pull requests just to fix up commit messages.

CONTRIBUTING.md

@@ -71,6 +71,22 @@ if we have to read the whole diff to figure out why you're contributing
 in the first place, you're less likely to get feedback and have your change
 merged in.
 
+In addition to outlining your thought process in the PR's description, please
+also try to document it in your commits. We welcome it if every commit has a
+description of why you have been doing your changes alongside with your
+reasoning why this is a good idea. The messages shall be written in
+present-tense and in an imperative style (e.g. "Add feature foobar", not "Added
+feature foobar" or "Adding feature foobar"). Lines should be wrapped at 80
+characters so people with small screens are able to read the commit messages in
+their terminal without any problem.
+
+To make it easier to attribute commits to certain parts of our code base, we
+also prefer to have the commit subject be prefixed with a "scope". E.g. if you
+are changing code in our merging subsystem, make sure to prefix the subject with
+"merge:". The first word following the colon shall start with an lowercase
+letter. The maximum line length for the subject is 70 characters, preferably
+shorter.
+
 If you are starting to work on a particular area, feel free to submit a PR
 that highlights your work in progress (and note in the PR title that it's
 not ready to merge). These early PRs are welcome and will help in getting