Merge pull request #8985 from prometheus/richih/pr_template

Add PR template

.github/PULL_REQUEST_TEMPLATE.md

@@ -1,6 +1,8 @@
 <!--
     Don't forget!
 
+    - Please sign CNCF's Developer Certificate of Origin and sign-off your commits by adding the -s / --sign-off flag to `git commit`. See https://github.com/apps/dco for more information.
+
     - 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.