add PR template

.github/pull_request_template.md

@@ -0,0 +1,20 @@
+## Description
+
+<!-- Please describe your pull request here. -->
+
+## Pull request checklist
+
+- [ ] add tests
+- documentation
+    - [ ] add docstrings
+    - [ ] if a module is new: add API reference file for Sphinx in `docs/source/api-reference`
+    - [ ] check API doc of Sphinx - build and show with `make sphinx && make open-sphinx`
+- [ ] fix checks
+    - start locale checks with `make check`
+    - fix formatting with `make format`
+- Python type info
+    - [ ] check and improve Python type info
+    - [ ] double check if return types are specified - inclusing `-> None`
+- [ ] check TODO comments in code
+- [ ] check FIXME comments in code
+- [ ] check if the copyright header needs an update