Specify where onCheckEmail should be defined

I guess `onCheckEmail` should be defined inside the page code, so I updated the document accordingly. Please correct me if my suggestion is wrong.

README.md

@@ -170,7 +170,7 @@ The User plugin displays AJAX error messages in a simple ``alert()``-box by defa
 
 ### Checking if a login name is already taken
 
-Here is a simple example of how you can quickly check if an email address / username is available in your registration forms. First create an AJAX handler to check the login name, here we are using the email address:
+Here is a simple example of how you can quickly check if an email address / username is available in your registration forms. First, inside the page code, define the following AJAX handler to check the login name, here we are using the email address:
 
  public function onCheckEmail()
  {