github: enhance pull request template (port from master)

.github/PULL_REQUEST_TEMPLATE

@@ -1,8 +0,0 @@
-Thank you for contributing code to Borg; your help is appreciated!
-
-Before you submit a pull request, please make sure it complies with the
-guidelines in our documentation:
-
-https://borgbackup.readthedocs.io/en/stable/development.html#contributions
-
-**Please remove the text above before submitting your pull request.**

.github/PULL_REQUEST_TEMPLATE.md

@@ -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