Add docs for after_enqueue hook

docs/HOOKS.md

@@ -66,6 +66,9 @@ An unnamed hook (`before_perform`) will be executed first.
 
 The available hooks are:
 
+* `after_enqueue`: Called with the job args after a job is placed on the queue.
+  Any exception raised propagates up to the code which queued the job.
+
 * `before_perform`: Called with the job args before perform. If it raises
   `Resque::Job::DontPerform`, the job is aborted. If other exceptions
   are raised, they will be propagated up the the `Resque::Failure`
@@ -99,6 +102,13 @@ look something like this.
 
 Modules are even better because jobs can use many of them.
 
+    module ScaledJob
+      def after_enqueue_scale_workers(*args)
+        Logger.info "Scaling worker count up"
+        Scaler.up! if Redis.info[:pending].to_i > 25
+      end
+    end
+
     module LoggedJob
       def before_perform_log_job(*args)
         Logger.info "About to perform #{self} with #{args.inspect}"
@@ -115,6 +125,7 @@ Modules are even better because jobs can use many of them.
     class MyJob
       extend LoggedJob
       extend RetriedJob
+      extend ScaledJob
       def self.perform(*args)
         ...
       end