We did not have an official guideline on how commit messages are supposed to
be formatted.
Inofficially we used something like [compositor] or [platforms/drm] as tags on
the subject line. But the square brackets in the beginning can lead to issues
with certain tools.
This patch specifies an official guideline which is mostly based on the one
developed in the Angular project:
https://github.com/angular/angular/blob/master/CONTRIBUTING.md#commit
Using the existing guideline of another big open source project mostly
unchanged hopefully allows us to set this guideline once and without major
changes afterwards.
Some details had to be changed though to take account of KWin's different
programming language and internal structure.