Add guide on how to add a field to the User model

[gogl92]

What steps will reproduce the problem?

It isn't documented how we should add a field to the User model although dektrium guide will apply the same.

What is the expected result?

As a developer I want to have the instruction in the docs on how to add a field to the User model.

This can be used, the examples might need to be updated. https://github.com/dektrium/yii2-user/blob/master/docs/adding-new-field-to-user-model.md

What do you get instead?

No guide for this topic supplied.

[TimoB2005]

There is an guide to override the User Model...

Follow this guide and you will extend the fields of the Model.

The only knowledge that you need here is a bit of Yii2.