Docs: Improve warning about whodunnit

[ci skip]

doc/warning_about_not_setting_whodunnit.md

@@ -0,0 +1,32 @@
+# The warning about not setting whodunnit
+
+After upgrading to PaperTrail 5, you see this warning:
+
+> user_for_paper_trail is present, but whodunnit has not been set. PaperTrail no
+> longer adds the set_paper_trail_whodunnit before_filter for you. Please add this
+> before_filter to your ApplicationController to continue recording whodunnit.
+
+## You want to track whodunnit
+
+Add the set_paper_trail_whodunnit before_filter to your ApplicationController.
+See the PaperTrail readme for an example (https://git.io/vrTYG).
+
+## You don't want to track whodunnit
+
+If you no longer want to track whodunnit, you may disable this
+warning by overriding user_for_paper_trail to return nil.
+
+```
+# in application_controller.rb
+def user_for_paper_trail
+  nil # disable whodunnit tracking
+end
+```
+
+## You just want the warning to go away
+
+To disable this warning for any other reason, use `skip_after_action`.
+
+```
+skip_after_action :warn_about_not_setting_whodunnit
+```

lib/paper_trail/frameworks/rails/controller.rb

@@ -98,8 +98,8 @@ module PaperTrail
             user_for_paper_trail is present, but whodunnit has not been set.
             PaperTrail no longer adds the set_paper_trail_whodunnit
             before_filter for you. Please add this before_filter to your
-            ApplicationController to continue recording whodunnit. See the
-            PaperTrail readme for an example.
+            ApplicationController to continue recording whodunnit. For more
+            information, please see https://git.io/vrTsk
           EOS
         end
       end