Update docs.

Author: vagrant

README.md

@@ -61,9 +61,10 @@ class User extends Model
 ## Notices
 
  - Nested transactions are supported. 
- Registered callbacks are deleted just after a nested transaction 
- rollback, and nested transaction commits happen only when 
- the top parent transaction is committed.
+ Whenever you roll back a nested transaction, all the containing
+ callbacks will be ignored and deleted immediately. Callbacks
+ happening inside a nested transaction will be fired whenever the
+ very very top parent transaction is committed. 
  - Commit callbacks will not be fired on rollback or exceptions.
  - Callbacks are immediately fired if their model's connection
  is not inside an active transaction.