updated upgrade file

UPGRADE.md

@@ -1,10 +1,18 @@
+Changes in version 2.4
+----------------------
+
+ * Deprecated the `assetic:dump --watch` option in favor of the new
+   `assetic:watch` command
+ * Added support for running `assetic:dump` across multiple processes using
+   `kriswallsmith/spork`
+
 Changes in version 2.1
 ----------------------
 
  * Friendlier exception message when using Assetic in a bundle that Assetic
    has not been configured to scan
- * Assets are no longer dumped to the filesystem when `cache:warmup` is run. 
+ * Assets are no longer dumped to the filesystem when `cache:warmup` is run.
    This can only be done using the `assetic:dump` command
  * Added support for GSS filter
- * Assetic's routes are now automatically added when `use_controller` is 
+ * Assetic's routes are now automatically added when `use_controller` is
    `true`