Update pull request template for new changelog system

Author: dstansby

.github/PULL_REQUEST_TEMPLATE.md

@@ -4,6 +4,6 @@ TODO:
 * [ ] Add unit tests and/or doctests in docstrings
 * [ ] Add docstrings and API docs for any new/modified user-facing classes and functions
 * [ ] New/modified features documented in `docs/user-guide/*.rst`
-* [ ] Changes documented in `docs/release-notes.rst`
+* [ ] Changes documented as a new file in `changes/`
 * [ ] GitHub Actions have all passed
 * [ ] Test coverage is 100% (Codecov passes)