Clarify and document support for nested fields in ChangeSet

[no-reply]

I ran into the following issue creating nested fields:

class MyResource < Valkyrie::Resource
  attribute :my_set
end

class MyChangeSet < Valkyrie::ChangeSet 
  collection :my_set do
     property :something
  end
end

r = MyResource.new(my_set: Struct.new(:something).new('value'))
MyChangeSet.new(r)
# NameError: undefined local variable or method `build_errors' for #<#<Class:0x000055de3fee2960>:0x000055de403e2a00> 

this seems to have something to do with an error in the setup but the NameError isn't helpful to diagnose, and appears to be a Reform bug(???).

i was eventually able to get this working, but it took a lot of trial and error. documenting and testing support for nested ChangeSets would be a huge help.