README.rdoc in composite_primary_keys-7.0.12 vs README.rdoc in composite_primary_keys-7.0.13

@@ -23,9 +23,13 @@
     Version 7.x is designed to work with ActiveRecord 4.1.x
     Version 6.x is designed to work with ActiveRecord 4.0.x
     Version 5.x is designed to work with ActiveRecord 3.2.x
     Version 4.x is designed to work with ActiveRecord 3.1.x
 
+Run the following command to list available versions:
+
+    gem list composite_primary_keys -ra
+
 == The basics
 
 A model with composite primary keys is defined like this:
 
   class Membership < ActiveRecord::Base