Add information about how generators work to the contribution guide

[composerinteralia]

Rails generators can be a bit of a mystery for folks unfamiliar with them. While trying to explain to somebody where some of the file manipulation methods come from it took me way too many steps to find the documentation I was hoping for:

1. The method wasn't defined in the suspenders generator I was looking at, so walked up the inheritance chain to Suspenders::Generators::Base
2. Not defined there either so continued up inheritance chain and searched for Rails::Generators::Base on https://api.rubyonrails.org/
3. Not defined there either, but mention of Thor::Actions
4. Clicked on lots of things to get to relevant thor documentation: https://github.com/erikhuda/thor -> https://github.com/erikhuda/thor/wiki -> https://www.rubydoc.info/github/erikhuda/thor -> https://www.rubydoc.info/github/erikhuda/thor/Thor/Actions

I probably should have gone to https://guides.rubyonrails.org/generators.html instead, but I didn't think of it until after.

Having a couple of links in the contributor guide will hopefully make this easier for my future self, and for others.

cc @fridaland

[stevepolitodesign]

A section has been added about how to test generators, but there's still an opportunity to link to https://guides.rubyonrails.org/generators.html