Added description/credit for recent changes.

Source/Changes.txt

@@ -1,6 +1,17 @@
 Listing of notable changes by release. More detail is usually found in the Git
 commit messages and/or the pull requests.
 
+OCMock 3.4.3 (unreleased)
+
+* Changed behaviour when a second mock takes over class method mocking from
+  an older mock for the same class. Now, stopMocking is no longer called on
+  the first mock, which means that the class generated by it stays around 
+  until stopMocking is called, directly or from dealloc; see issue #357 for
+  details.
+* Changed sequence in which steps are taken to swap out class methods, in
+  order to improve thread-safety; see issue #328 for details. (Lily Ballard)
+
+
 OCMock 3.4.2 (2018-06-25)
 
 * Skipping attempt to replace class methods on subclasses of NSManagedObject,