Add a bullet point to update the release checklist

Author: seisman

.github/ISSUE_TEMPLATE/4-release_checklist.md

@@ -47,6 +47,7 @@ assignees: ''
 - [ ] Announce the release on:
   - [ ] GMT [forum](https://forum.generic-mapping-tools.org/c/news/) (do this announcement first! Requires moderator status)
   - [ ] [ResearchGate](https://www.researchgate.net) (after forum announcement, add new version as research item via the **code** category, be sure to include the corresponding new Zenodo DOI)
+- [ ] Update release checklist template with any additional bullet points that may have arisen during the release
 
 ---