Added an example for an aggregate key as this was not clear.

Author: Oliver Lloyd

source/reference/aggregation.txt

@@ -423,6 +423,10 @@ The current pipeline operators are:
    a single field from the documents in the pipeline, a previously computed
    value, or an aggregate key made up from several incoming fields.
 
+   .. note::
+
+      An aggregate key takes the format: ``{myKeyName: { author:true, myOtherField:true , YetAnotherField:true}}``.
+
    Every group expression must specify an ``_id`` field.
    You may specify the ``_id`` field as a dotted
    field path reference, a document with multiple fields enclosed in