document the propper way for fixtures_replacement please

[StefanLiebenberg]

could you elaborate on replacing fixtures with machinist on this page please:

https://github.com/notahat/machinist/wiki/Installation

I cant find any propper way to do it other that this post... which might be wrong:

http://stackoverflow.com/questions/2123382/how-to-replace-fixture-generator-on-rails-3