Added a note about on_failure hooks to doc

The on_failure hook is already documented. I simply added a note that the
Reque::DirtyExit exception is also passed to the hook now.

Author: raykrueger

docs/HOOKS.md

@@ -86,7 +86,7 @@ The available hooks are:
   `Resque::Failure` backend.
 
 * `on_failure`: Called with the exception and job args if any exception occurs
-  while performing the job (or hooks).
+  while performing the job (or hooks), this includes Resque::DirtyExit.
 
 Hooks are easily implemented with superclasses or modules. A superclass could
 look something like this.