added user name password creation rules

pages/create-an-ecloud-account.md

@@ -40,6 +40,7 @@ toc: true
 
 * Fill the fields correctly ! This example will create an /e/ identity as John Doe <john.doe@e.email>
 * Do not use special characters in your username or password
+* Do not use a combination of uppercase and lower case alphabets in your username
 
 ![](images/your-e-id.png)