readme update for before_started

Author: virtualstaticvoid

README.md

@@ -308,7 +308,10 @@ module MyProcess
   # defines a process
   define_process do
 
-    # tasks, sub-process, etc.
+    # define task to execute on before
+    before_started :slack_notification
+
+    # usual tasks, sub-process, etc.
 
     # define task to execute on completion
     after_completed :further_process
@@ -318,6 +321,10 @@ module MyProcess
 
   end
 
+  def slack_notification
+    # ...
+  end
+
   def further_process
     # ...
   end