Add notes for issue templates that are only meant for project maintainers (#3714)

Author: seisman

.github/ISSUE_TEMPLATE/4-release_checklist.md

@@ -1,6 +1,6 @@
 ---
 name: PyGMT release checklist
-about: Checklist for a new PyGMT release.
+about: Checklist for a new PyGMT release. [For project maintainers only!]
 title: Release PyGMT vX.Y.Z
 labels: maintenance
 assignees: ''

.github/ISSUE_TEMPLATE/5-bump_gmt_checklist.md

@@ -1,6 +1,6 @@
 ---
 name: Bump GMT version checklist
-about: Checklist for bumping the minimum required GMT version.
+about: Checklist for bumping the minimum required GMT version. [For project maintainers only!]
 title: Bump to GMT X.Y.Z
 labels: maintenance
 assignees: ''