added link to MongoDB query tutorial

docs/advanced.rst

@@ -234,16 +234,17 @@ the returned data is completed with the specs of the selected fireworks::
     name: The coffee workflow
 
 The third example demonstrates even more complex use of a query for a DFT
-calculation of a water molecule. The query include a regular expression, and the
+calculation of a water molecule. The query includes a regular expression and the
 workflow metadata in the filter section. Additionally, more than one firework is
 selected and for one firework specific updates (from all updates) are selected.
 
-All queries are mongo queries and have the pymongo syntax.
+For a detailed tutorial on queries it is referred to the MongoDB query tutorial
+(select there your programming language): 
 
+https://docs.mongodb.com/manual/tutorial/query-documents/
 
-Use FilePad to store files
---------------------------
-
+The queries used here are pymongo query, i.e. have Python syntax, and are
+serialized to YAML and to JSON:
 
 
 Not every feature of FireWorks is covered in this tutorial. Please visit the