Projection optimization note (#460)

Author: jordan-smith721

source/whats-new.txt

@@ -58,6 +58,9 @@ The 3.2 driver release includes the following new features:
 
      EnumRepresentationConvention(BsonType representation, bool topLevelOnly);
 
+- Optimizes client-side projections to retrieve only the required fields from
+  the server, rather than retrieving all fields and then filtering them.
+
 .. _csharp-version-3.1:
 
 What's New in 3.1