Add contributing guideline about merge requests

CONTRIBUTING.rst:
Elaborate on when merge requests are merged using `--no-ff` or `--ff`.

CONTRIBUTING.rst

@@ -27,6 +27,20 @@ explaining it.
 The changelog entry needs to be added to the unreleased section at the top, as that section is used for the next
 release.
 
+Merge requests and signed commits
+=================================
+
+Merge requests are not required to contain signed commits (using ``git commit -S`` - see `man 1 git-commit
+<https://man.archlinux.org/man/git-commit.1>`_).
+The project maintainers may rebase a given merge request branch at their discretion (if possible), which may remove
+signed commits.
+
+The tip of the project's default branch is required to be a signed commit by the project maintainers.
+For external contributors this means, that their merge request will be merged using ``--no-ff`` (see `man 1 git-merge
+<https://man.archlinux.org/man/git-merge.1>`_) in a signed merge commit, while contributions by the project maintainers
+may be merged using ``--ff`` when the top-most commit of the source branch is signed by a valid PGP key of the given
+maintainer.
+
 Testing
 =======