Update issue templates

Author: mwcraig

.github/ISSUE_TEMPLATE/GITHUB_ADMIN.md

@@ -1,9 +1,10 @@
 ---
 name: Astropy Github Organisation Administration
 about: Request that an Astropy GitHub org admin performs some action.
-labels: [github-admin]
-assignees:
-  - astropy/coordinators
+title: ''
+labels: github-admin
+assignees: ''
+
 ---
 
 ### Description

.github/ISSUE_TEMPLATE/add-maintainer-checklist.md

@@ -0,0 +1,10 @@
+---
+name: Add maintainer checklist
+about: Reminder to check list of steps when modifying team
+title: ''
+labels: ''
+assignees: ''
+
+---
+
+If you are adding or removing someone from a team please check that the list of items here has been completed: https://github.com/astropy/astropy-project/blob/main/policies/adding-new-role-members.md