Added section for Sharding and moved notes under it

Author: IanWhalen

source/release-notes/2.2.txt

@@ -241,6 +241,21 @@ In Sharded Output of MapReduce Operation, Choose Initial Split Points
 
 :issue:`SERVER-4521`
 
+Sharding Improvements
+~~~~~~~~~~~~~~~~~~~~~
+
+Fixed Issue with some Queries against Recently Sharded Collections
+``````````````````````````````````````````````````````````````````
+
+:issue:`SERVER-4171`
+
+Index on Shard Keys Can Now Be a Compound Index
+```````````````````````````````````````````````
+
+:issue:`SERVER-1506`
+
+Documentation: :ref:`sharding-shard-key-indexes`
+
 Additional Improvements
 ~~~~~~~~~~~~~~~~~~~~~~~
 
@@ -264,11 +279,6 @@ Tracking i/o Time and Lock Time in Profiler
 
 :issue:`SERVER-3483`
 
-Fixed Issue with some Queries against Recently Sharded Collections
-``````````````````````````````````````````````````````````````````
-
-:issue:`SERVER-4171`
-
 Padding Specifiable on Compact Command
 ``````````````````````````````````````