Add documentation for on option.

README.md

@@ -7,7 +7,7 @@ There's an excellent [Railscast on implementing Undo with Paper Trail](http://ra
 
 ## Features
 
-* Stores every create, update and destroy.
+* Stores every create, update and destroy (or only the lifecycle events you specify).
 * Does not store updates which don't change anything.
 * Allows you to specify attributes (by inclusion or exclusion) which must change for a Version to be stored.
 * Allows you to get at every version, including the original, even once destroyed.
@@ -169,6 +169,16 @@ Here's a helpful table showing what PaperTrail stores:
 PaperTrail stores the values in the Model Before column.  Most other auditing/versioning plugins store the After column.
 
 
+## Choosing Lifecycle Events To Monitor
+
+You can choose which events to track with the `on` option.  For example, to ignore `create` events:
+
+    class Article < ActiveRecord::Base
+      has_paper_trail :on => [:update, :destroy]
+    end
+
+
+
 ## Choosing Attributes To Monitor
 
 You can ignore changes to certain attributes like this:

lib/paper_trail/has_paper_trail.rb

@@ -11,6 +11,8 @@ module PaperTrail
       # the model is available in the `versions` association.
       #
       # Options:
+      # :on           the events to track (optional; defaults to all of them).  Set to an array of
+      #               `:create`, `:update`, `:destroy` as desired.
       # :class_name   the name of a custom Version class.  This class should inherit from Version.
       # :ignore       an array of attributes for which a new `Version` will not be created if only they change.
       # :only         inverse of `ignore` - a new `Version` will be created only for these attributes if supplied