Add the CHANGELOG requirement in the PR template (#1679)

The comment would be an example. Related to #1662. This would be a
temporary solution until we can have a robust check.

### New Pull Request Checklist

- [ ] I have read the
[Documentation](https://pub.dev/documentation/dio/latest/)
- [ ] I have searched for a similar pull request in the
[project](https://github.com/cfug/dio/pulls) and found none
- [ ] I have updated this branch with the latest `main` branch to avoid
conflicts (via merge from master or rebase)
- [ ] I have added the required tests to prove the fix/feature I'm
adding
- [ ] I have updated the documentation (if necessary)
- [ ] I have run the tests without failures
- [ ] I have updated the `CHANGELOG.md` in the corresponding package

### Additional context and info (if any)

<!-- Provide more context and info about the PR. -->

Signed-off-by: Alex Li <[email protected]>

.github/PULL_REQUEST_TEMPLATE.md

@@ -8,6 +8,7 @@
 - [ ] I have added the required tests to prove the fix/feature I'm adding
 - [ ] I have updated the documentation (if necessary)
 - [ ] I have run the tests without failures
+- [ ] I have updated the `CHANGELOG.md` in the corresponding package
 
 ### Additional context and info (if any)