[github] mention major_change label (#9181)

* [github] mention `major_change` label

* Update .github/PULL_REQUEST_TEMPLATE.md

Co-authored-by: maxime mouial <hush-hush@users.noreply.github.com>

Co-authored-by: maxime mouial <hush-hush@users.noreply.github.com>

.github/PULL_REQUEST_TEMPLATE.md

@@ -56,6 +56,7 @@ Note: Adding GitHub labels is only possible for contributors with write access.
 
 - [ ] If known, an appropriate milestone has been selected; otherwise the `Triage` milestone is set.
 - [ ] The appropriate `team/..` label has been applied, if known.
+- [ ] Use the `major_change` label if your change either has a major impact on the code base, is impacting multiple teams or is changing important well-established internals of the Agent. This label will be use during QA to make sure each team pay extra attention to the changed behavior. For any customer facing change use a releasenote.
 - [ ] A [release note](https://github.com/DataDog/datadog-agent/blob/main/docs/dev/contributing.md#reno) has been added or the `changelog/no-changelog` label has been applied.
 - [ ] Changed code has automated tests for its functionality.
 - [ ] Adequate QA/testing plan information is provided if the `qa/skip-qa` label is not applied.