warning about partition fields (#277)

Author: kyuan-mongodb

source/tutorial/add-dataset-pipeline.txt

@@ -165,6 +165,11 @@ Create a Pipeline from the |service| UI
       </core/document/#dot-notation>`. You can't specify fields inside 
       an array. The specified fields are used to partition your data. 
 
+      .. warning::
+
+         You can't specify field names that contain periods (``.``) for
+         partitioning.
+
       The most frequently queried fields should be listed towards the 
       top because they will have a larger impact on performance and c
       ost than fields listed lower down the list. The order of fields