Added a note about BC break for DateType into UPDATE.md file

UPDATE.md

@@ -6,6 +6,11 @@ one. It only discusses changes that need to be done when using the "public"
 API of the framework. If you "hack" the core, you should probably follow the
 timeline closely anyway.
 
+beta3 to beta4
+--------------
+
+* Form DateType parameter single-text changed to single_text 
+
 beta2 to beta3
 --------------
 
@@ -513,4 +518,4 @@ PR8 to PR9
                     jar:      "/path/to/yuicompressor.jar"
                 my_filter:
                     resource: "%kernel.root_dir%/config/my_filter.xml"
-                    foo:      bar
+                    foo:      bar