Clarify the roles of resource service methods

[jensljungblad]

In particular, clarify the roles of resources_relation and resources, perhaps with a rename. They were previously called begin_of_association_chain and end_of_association_chain which were a bit of a mouthful but perhaps more clear. Those names originally came from https://github.com/activeadmin/inherited_resources.

One issue is that the examples we have in the readme are not very good. Both could be implemented using the other function instead. So we should elaborate a bit on why both are needed. Currently, I can't remember 😳