Add information about joins in hanami/rom

[davydovanton]

I think it's an important theme for guides. That's why I suggest to add this. We can use this SO question for example:
https://stackoverflow.com/questions/43080678/join-query-in-hanami-model

Reference to #343

[davydovanton]

/cc @hanami/core

[alvir]

I was confused with query documentation in guides. I think It will be helpful to add info about rom-sql and Sequel.

[mereghost]

Some guides would be nice, specially to the features where we meddle with rom (associations and the like).

Covering all the queries would make our docs coupled to rom's docs, so it would be better to point users to rom docs.

[cveneziani]

What about this issue now, since Pull Request #386 has been merged?

[davydovanton]

@cveneziani hey! you can take my old PR and update it if you want #344 :)

[cveneziani]

Well, as @mereghost mentioned. Shall we copy the doc from ROM or just point out to ROM doc? And if we copy, where shall we stop?

What's you point of view?