Docs: Mention that states need to be registered in forRoot()

[ceelian]

If you follow the docs on the website top to bottom like a tutorial, it is not mentioned (therefore some might forget) that the BookState which is used in the example needs to be added to the LoonaModule.forRoot:

imports: [LoonaModule.forRoot([BookState])],

Should possibly be added at some point in the docs.

[rafagsiqueira]

@ceelian Thank you for this! I had to go through the source code to figure that out.