Update README with working strong params example
Update README with working strong params example

The API for `Devise::ParameterSanitizer` changed here: https://github.com/plataformatec/devise/commit/e79201aef86e048ed38e2181675458bc3b9da2a1#diff-3d9087a6e7f173db4dd513c2139254ec

The new example worked for me with Devise 4.2.0 on Rails 5.0.0.1
scambra/devise_invitable,scambra/devise_invitable