title: How to add email address as child in Firebase?
body: <p>I am working on an Android App where I want a Firebase database like 
  <a href="https://i.stack.imgur.com/12YoL.jpg" rel="noreferrer"><img src="https://i.stack.imgur.com/12YoL.jpg" alt="enter image description here"></a></p>
  
  <p>Basically, I want to verify email before creating a user on the app. Whenever a user registers in my app, First it will be checked whether the email address is available in the <strong>users</strong> database reference. if available, then only he is allowed to register.
  But I found that Firebase doesn't allow an email as a child. So How can I do it? Any other suggestion for achieving this in Firebase Database.</p>