title: validating presence of email address (can't be blank)- Ruby on Rails
body: <p>Pretty new at all this. I have a simple form for users to enter a couple pieces of information and then input their email address and push the submit button. I want to make it mandatory that they have to fill out their email address in order to push the submit button. If they don't fill out their email address they should get an error message on the email box that says the email can't be blank. I know this is super simple but I need exact help on where to put what code. I've been researching this all night and know that part of the code should go in the application_controller and other should go in the html file where the actual text_field:email is. 
  I'd be grateful if someone could clearly tell me what the necessary steps are for doing this. Thanks! </p>