[api] add information on how to assign admin users

sda/cmd/api/api.md

@@ -118,3 +118,17 @@ Admin endpoints are only available to a set of whitelisted users specified in th
   - `200` Query execute ok.
   - `401` Token user is not in the list of admins.
   - `500` Internal error due to DB failure.
+
+#### Configure Admin users
+
+The users that should have administrative access can be set in two ways:
+
+- As a comma separated list of user identifiers asigned to: `admin.users`.
+- As a JSON file containg a list of the user identities, the path to the file is assigned to: `admin.configFile`. This is the recommended way.
+
+```json
+[
+"[email protected]"
+"[email protected]"
+]
+```