Add two extra checks to the pull request template.

These should help catch common things missed in PRs that mean that e.g. documentation gets out of sync with reality.

Author: daj-code

.github/PULL_REQUEST_TEMPLATE.md

@@ -15,4 +15,6 @@ Anything you think will be useful for reviewers.
 These aren't hard requirements, just guidelines
 
 - [ ] New/modified Rust code formatted with `cargo fmt`
+- [ ] Documentation and `README.md` updated for this change
+- [ ] `CHANGELOG.md` updated for this change