explain error messages in README

README.md

@@ -20,3 +20,13 @@ Put something like this in your models:
     validates :count, :unit => {:allowed_types => :unitless}
     validates :width, :unit => {:allowed_types => [:mass, :length]}
 
+### Error messages
+
+The error message upon failure can be overridden by passing the option `message` to the unit validation:
+
+    validates :quantity, :unit => {:message => 'please be so kind as to enter a physical quantity'}
+
+If not supplied, a localised message is shown.
+Translations are kept in [localeapp#6170](http://www.localeapp.com/projects/6170) - please feel free to
+add a new translation or correct something.
+