If your template doesn't already feature an email address field, you can select the Email address element from the left-hand palette and drag it in. Click on Type question here to then enter your text for it.
This element is already prepared to accept an email address, therefore email validation is automatically configured (but you do have the choice to write your own validation error message).
Enabling data field mapping
As well as the usual configuration settings, you have the choice to Enable data field mapping. Checking this box produces the Data field mapping tab, allowing you to add or update respondents' details to an address book through through their email address. You can either choose an existing address book in your account in which to do this, or you can create an entirely new address book.
Enabling data field mapping will mean you can then map other questions to a data field, meaning your respondent's answer will be stored under that data field in your address book, enriching your contact data.