Add Sandi Metz’ rules for developers

We’ve determined that using Metz’ rules has been a success, so I’ve pulled these
out of the technical guidelines document for our project which was using them.

[Ruby Rogues Book
Club](http://rubyrogues.com/087-rr-book-clubpractical-object-oriented-design-in-ruby-with-sandi-metz/)

@halogenandtoast @croaky @derekprior @mjankowski @mike-burns