Add pull request template for changelist (#67)

Author: lagru

.github/PULL_REQUEST_TEMPLATE.md

@@ -0,0 +1,13 @@
+<!--
+- Reference relevant issues or related pull requests with their URL / #<number>.
+- Use `pre-commit` to check and format code.
+-->
+
+
+## Release note
+
+For maintainers and optionally contributors, please refer to [changelist's README](https://github.com/scientific-python/changelist) on how to document this PR for the release notes.
+
+```release-note
+...
+```