Add/Remove admins (#337)

docs/administration/manage-admins.md

@@ -0,0 +1,11 @@
+# Manage admins
+
+You can add and remove Opengist admins from the CLI.
+
+```bash
+./opengist admin toggle-admin <username>
+```
+```bash
+$ ./opengist admin toggle-admin thomas
+User thomas admin set to true
+```