Mention release drafter labels in PR template checklist

It's important to set proper labels for PRs so that later on
when new release will be prepared, release drafter plugin could generate
concise release notes.

.github/PULL_REQUEST_TEMPLATE.md

@@ -32,6 +32,7 @@ Before | After
 - [ ] Git commits follow [best practices](https://chris.beams.io/posts/git-commit/) <!-- mandatory -->
 - [ ] Build passes in Jenkins <!-- mandatory -->
 - [ ] Appropriate tests or explanation to why this change has no tests <!-- mandatory -->
+- [ ] Pull Request is marked with appropriate label (see `.github/release-drafter.yml`) <!-- mandatory -->
 - [ ] JIRA issue is well described (problem explanation, steps to reproduce, screenshots) <!-- optional -->
 - [ ] For dependency updates: links to external changelogs and, if possible, full diffs