Add documentation for external pillar cmd json

salt/pillar/cmd_json.py

@@ -1,5 +1,23 @@
 """
 Execute a command and read the output as JSON. The JSON data is then directly overlaid onto the minion's Pillar data.
+
+
+Configuring the CMD_JSON ext_pillar
+====================================
+
+Set the following Salt config to setup cmd json result as external pillar source:
+
+.. code-block:: yaml
+
+  ext_pillar:
+    - cmd_json: 'echo {\"arg\":\"value\"}'
+
+This will run the command ``echo {arg: value}`` on the master.
+
+
+Module Documentation
+====================
+
 """
 
 import logging