Merge branch 'info_about_namespaces' into 'master'

Info about namespaces

Added info about namespaces in groups.

Because of tweet https://twitter.com/kaystrobach/status/603912904757420032

See merge request !1835

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:
+```
+/groups?per_page=100
+```
+
+And to switch pages add: 
+```
+/groups?per_page=100&page=2
+```
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