DOCSP-44733 - Immutable-collection serializers (#341)

Author: mongoKart

source/whats-new.txt

@@ -19,6 +19,7 @@ What's New
 
 Learn what's new in:
 
+* :ref:`Version 3.1 <csharp-version-3.1>`
 * :ref:`Version 3.0 <csharp-version-3.0>`
 * :ref:`Version 2.30 <csharp-version-2.30>`
 * :ref:`Version 2.29 <csharp-version-2.29>`
@@ -29,10 +30,21 @@ Learn what's new in:
 * :ref:`Version 2.24 <version-2.24>`
 * :ref:`Version 2.23 <version-2.23>`
 * :ref:`Version 2.22 <version-2.22>`
-* :ref:`Version 2.21 <version-2.21>`
 
 .. _upcoming-breaking-changes:
 
+.. _csharp-version-3.1:
+
+What's New in 3.1
+-----------------
+
+The 3.1 driver release includes the following new features:
+
+- Adds new default serializers for immutable collections in the
+  `System.Collections.Immutable <https://learn.microsoft.com/en-us/dotnet/api/system.collections.immutable?view=net-8.0>`__
+  namespace. The driver can now serialize ``ImmutableArray`` objects, and serialization
+  for other immutable collections is more memory efficient.
+
 .. _csharp-version-3.0:
 
 What's New in 3.0