Adds information on what's new label to PR template (grafana#57594)

.github/PULL_REQUEST_TEMPLATE.md

@@ -14,6 +14,8 @@ Thank you for sending a pull request! Here are some tips:
 
 6. Name your PR as "<FeatureArea>: Describe your change", e.g. Alerting: Prevent race condition. If it's a fix or feature relevant for the changelog describe the user impact in the title. The PR title is used to auto-generate the changelog for issues marked with the "add to changelog" label.
 
+7. If your PR content should be added to the What's New document for the next major or minor release, add the **add to what's new** label to your PR. Note that you should add this label to the main PR that introduces the feature; do not add this label to smaller PRs for the feature.
+
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 **What is this feature?**
@@ -28,7 +30,6 @@ Thank you for sending a pull request! Here are some tips:
 
 [Add information on what kind of user the feature is for.]
 
-
 **Which issue(s) does this PR fix?**:
 
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