added PR template

Author: mlodic

.github/pull_request_template.md

@@ -0,0 +1,6 @@
+# Checklist
+
+- [ ] The pull request is for the branch `main`
+- [ ] If I added new documentation related to a new Pull Request that I created, I have added a reference to the related Pull Request
+  - [ ] If the new documentation will be added in a future release, please add a reference like `_Available from version > x.x.x_` where x.x.x is the current version number
+- [ ] If the additional documentation is a fix, you should directly push to the `main` branch and do not open this PR.