added info about namespaces

doc/api/groups.md

@@ -176,3 +176,17 @@ Parameters:
 
 - `id` (required) - The ID or path of a user group
 - `user_id` (required) - The ID of a group member
+
+## Namespaces in groups
+
+By default, groups only get 20 namespaces at a time because the API results are paginated.
+
+To get more (up to 100), pass the following as an argument to the API call
+```
+per_page=100
+```
+
+And to switch pages add 
+```
+page=2
+```