update PR template with changelog note

.github/PULL_REQUEST_TEMPLATE.md

 ### Required for all PRs:
 
-- [ ] CHANGELOG.md updated
+- [ ] CHANGELOG.md updated (we recommend not updating this until the PR has been approved by a maintainer)
 - [ ] Sign [CLA](https://influxdata.com/community/cla/) (if not already signed)
 - [ ] README.md updated (if adding a new plugin)