Mention -D js-flatten in the change log.

doc/CHANGES.txt

@@ -11,6 +11,7 @@
 	all : allow recursive type parameter constraints
 	all : allow @:fakeEnum on abstracts
 	all : allow Class.new
+	js : added -D js-flatten
 	macro : allowed $v{(c:Float|Int|String)}
 	macro : resolve error line number in external files