Add to documentation: nova-checkboxes for relationships

[blomdahldaniel]

Hi! I just found this great article that explained how to use your great package for relationships.

https://ivanmijatovic.com/laravel-nova-belongstomany-relationship-as-checkbox

Works for both BelongsTo, BelongsToMany and MorphToMany. So very very powerful stuff! This was the exact use case for me, that is why I searched for your repo. I suggest to maybe add this information to your documentation so that others can use this as well?