Add more information to Rails 4 in README

I suspect people who encounter this bug will search for the ActiveRecord exception. Adding the message to the README will lead them directly to the best source of the fix (I came through collectiveidea#730).

README.md

@@ -63,9 +63,13 @@ Development
 In development mode, if you are using Rails 3.1+, your application code will automatically reload every 100 jobs or when the queue finishes.
 You no longer need to restart Delayed Job every time you update your code in development.
 
-Rails 4
-=======
-If you are using the protected_attributes gem, it must appear before delayed_job in your gemfile.
+Rails 4.x
+=========
+If you are using the protected_attributes gem, it must appear before delayed_job in your gemfile. If your jobs are failing with:
+
+     ActiveRecord::StatementInvalid: PG::NotNullViolation: ERROR:  null value in column "handler" violates not-null constraint
+
+then this is the fix you're looking for.
 
 Upgrading from 2.x to 3.0.0 on Active Record
 ============================================