Update README.md

    4.1) **Using Retrofit**

    4.1.1) Before using this library (using retrofit)

    If you have an interface like this:

    4.1.1) Before using this single sign on library, your interface for your retrofit API might look like this:

    ```java

    public interface API {

    }

    ```

    You might be using a wrapper around your retrofit api to handle the instantiation etc. The code might look like this:

    You might instantiate your `API` by using something like the following code: 

    ```java

    public class ApiProvider {