I need to add some extra fields to registration form and the only information I found related is this. That's not clear at all so for better understand how this works will be nice if someone could help me to place lets say a phone number field that would be required and a country field (which would be ideal get countries that are already in the database ) this could be optional, the idea is to understand how to fit this on my controller and where exactly I mean plugin, component or should place it in somewhere where registration extended, any help?

Added this to the docs:

https://github.com/rainlab/user-plugin#using-a-login-name

Enjoy