PULL_REQUEST_TEMPLATE.md: add release notes (#2801)

We'd like to experiment with putting these in the pull request because
they are much easier to edit.
## Checklist

- [ ] I have added
[tests](https://www.cursorless.org/docs/contributing/test-case-recorder/)
- [ ] I have updated the
[docs](https://github.com/cursorless-dev/cursorless/tree/main/docs) and
[cheatsheet](https://github.com/cursorless-dev/cursorless/tree/main/cursorless-talon/src/cheatsheet)
- [ ] I have not broken the cheatsheet

Author: phillco

.github/PULL_REQUEST_TEMPLATE.md

@@ -5,3 +5,5 @@
 - [ ] I have added [tests](https://www.cursorless.org/docs/contributing/test-case-recorder/)
 - [ ] I have updated the [docs](https://github.com/cursorless-dev/cursorless/tree/main/docs) and [cheatsheet](https://github.com/cursorless-dev/cursorless/tree/main/cursorless-talon/src/cheatsheet)
 - [ ] I have not broken the cheatsheet
+
+## Release notes