Add PR template

Signed-off-by: Richard Hartmann <richih@richih.org>

.github/PULL_REQUEST_TEMPLATE.md

@@ -1,6 +1,8 @@
 <!--
     Don't forget!
 
+    - Sign your commits and our DCO:  https://docs.github.com/en/github/authenticating-to-github/managing-commit-signature-verification/signing-commits
+
     - If the PR adds or changes a behaviour or fixes a bug of an exported API it would need a unit/e2e test.
 
     - Where possible use only exported APIs for tests to simplify the review and make it as close as possible to an actual library usage.