Fixed Entity Framework Core integration steps and explained integration more thoroughly

[hexagonite]

Hi, I tried to use this library in my project and I've found that the Entity Framework Core integration code snippet was missing parameterless constructors, as I got this exception during creation of migrations:

System.InvalidOperationException: "Cannot create an instance of value converter type 'UlidToStringConverter'. Ensure that the type can be instantiated and has a parameterless constructor, or use the overload of 'HasConversion' that accepts a delegate."

I used the same solution as here.

Moreover, I added further steps on how to use those ValueConverters so that the library is easy to use for experienced and inexperienced programmers alike.

Please let me know if there are any issues with my PR, I will gladly make all the necessary changes. I'm new to contributing to public repositories :)

[neuecc]

Thank you! Very good!