added grains state instructions

salt/states/grains.py

@@ -1,5 +1,20 @@
 '''
-Manage grains on the minion. This state allows for grains to be set and unset
+Manage grains on the minion.
+============================
+
+This state allows for grains to be set. If a grain with the 
+given name exists, its value is updated to the new value. If 
+a grain does not yet exist, a new grain is set to the given
+value. Grains set or altered this way are stored in the 'grains'
+file on the minions, by default at: /etc/salt/grains 
+
+Note: This does NOT override any grains set in the minion file.
+
+.. code-block:: yaml
+
+    cheese:
+      grain.present:
+        - value: edam
 '''
 
 def present(name, value):