Add checklist item (#5715)

* Add checklist item

* Update .github/ISSUE_TEMPLATE/release.md


Co-authored-by: Mara Robin B. <mara@broda.me>

polkadot/.github/ISSUE_TEMPLATE/release.md

@@ -48,4 +48,5 @@ candidate branch or started an additional release candidate branch (rc-2, rc-3,
     notes](https://github.com/paritytech/polkadot/blob/master/doc/release-checklist.md#release-notes)
 - [ ] Check that [build artifacts](https://github.com/paritytech/polkadot/blob/master/doc/release-checklist.md#build-artifacts) have been added to the
     draft-release
-- [ ] Check that all items listed in the [milestone](https://github.com/paritytech/polkadot/milestones) are included in the release.
\ No newline at end of file
+- [ ] Check that all items listed in the [milestone](https://github.com/paritytech/polkadot/milestones) are included in the release.
+- [ ] Ensure that no `freenotes` were added into the release branch after the latest generated RC