How to Have Yahoo! Mail Add New Contacts to Your Address Book Automatically

In a jiffy, a computer can only do so much: whatever fits between two ticks of its clock — which typically happen a few milliseconds apart.
Adding a new contact should be possible in a Yahoo! Mail jiffy or two, and it won't take you a jiffy at all:
›› Yahoo! Mail can build your email address book: here's how to have Yahoo! Mail add everybody you mail to your "Contacts" automatically.