add a PR template

.github/PULL_REQUEST_TEMPLATE.md

+
+## Description
+
+**Please provide a short abstract motivating your pull request.**
+
+
+## Breaking changes
++ *list all breaking changes that this PR introduces*
+
+## Backwards-compatible changes
++ *list all backwards-compatible changes that this PR introduces*
+
+## Details
+
+**For non-trivial PRs, please explain the background and implementation details.**
+
+## PR checklist
++ [ ] are the changes covered by tests?
++ [ ] re-run example notebooks if they are effected in a relevant way
++ [ ] **mention the PR in the RELEASE-NOTES.md**