Adds extra documentation around how to specify the serializable looku…

[ZempTime]

…p across different places in rails applications.

Related to: jsonapi-rb/jsonapi-rails#68

Here's what I added, so you don't have to imagine it from markdown or run a bunch of commands on your computer to see it:

[gregjoy1]

Not a jsonapi-rb merge rights holder, just wanted to say that although this is a weird place for me to find documentation. I found this super helpful, thank you @ZempTime and I hope this gets merged :)

One trivial note would be that this could mention that this could be achieved with the jsonapi.rb initializer that we are told to generate with rails generate jsonapi:initializer. Initializer template can be found - https://github.com/jsonapi-rb/jsonapi-rails/blob/master/lib/generators/jsonapi/initializer/templates/initializer.rb

Thanks again @ZempTime :)

[beauby]

Thanks a lot for this, sorry it took so long to merge.