Readme documentation on using `accepts_nested_attribute_for` with Strong Parameters

Copied over documentation from http://edgeapi.rubyonrails.org/classes/ActionController/StrongParameters.html

I was banging my head against the wall for a while figuring out why the nested parameter documentation currently on the site was not working for me. I thought this would save others the headache.

I literally just copied over documentation from the StrongParameters doc on edge rails.