github: enhance pull request template, fixes #9334

Author: mr-raj12

.github/PULL_REQUEST_TEMPLATE

@@ -0,0 +1,18 @@
+<!--
+Thank you for contributing to BorgBackup!
+
+Please make sure your PR complies with our contribution guidelines:
+https://borgbackup.readthedocs.io/en/latest/development.html#contributions
+-->
+
+## Description
+
+<!-- What does this PR do? Reference any related issues with "fixes #XXXX". -->
+
+
+## Checklist
+
+- [ ] PR is against `master` (or maintenance branch if only applicable there)
+- [ ] New code has tests and docs where appropriate
+- [ ] Tests pass (run `tox` or the relevant test subset)
+- [ ] Commit messages are clean and reference related issues