Add custom I18n example on doc

Maybe it could be part of the template generation.
This pull request avoid the user search into the code to know the I18n key.

Author: wbotelhos

Diff for: README.md

@@ -499,6 +499,15 @@ end
 Note that you *will eventually have data corruption* if you disable advisory locks, write to your
 database with multiple threads, and don't provide an alternative mutex.
 
+## I18n
+
+You can customize error messages using [I18n](http://guides.rubyonrails.org/i18n.html):
+
+```yaml
+en-US:
+  closure_tree:
+    loop_error: Your descendant cannot be your parent!
+```
 
 ## FAQ