Mention the User association setup in the docs

[adamlofting]

While setting up noticed for the first time today, and following the docs in the README, it took me a little while to work out that we need to define the has_many :notifications, as: :recipient relation on the User model.

I later found this is mentioned in the screencast, but it's not in the docs and adding an explanation about this could help new users.

Overall though, I'm enjoying the design of noticed, so thanks for all the work that has gone into it already.