Configuring "@" CNAME record in GoDaddy control panel

In short, you can't make the @ record a CNAME without deleting all other resource records for @, and you can't do that since some (like the NS records) are required for proper DNS functionality. This is one reason why providers such as Heroku tell you not to use naked domain names.

You will need a host to perform the HTTP redirection from example.com to www.example.com for you, to which you will point A (and AAAA) record for @.

If your DNS is hosted with GoDaddy, then they have a free service that will do this for you. In your GoDaddy domain manager, look on the left hand side for "Forwarding" and click "Manage". Then set it to forward example.com to www.example.com and update your DNS to support the change. You should leave the Advanced Options at their defaults.