Docs Command Line Arguments for Generators

[kevinhughes27]

Summary

This PR adds a documentation section explaining how to add command line arguments to Rails generators. This functionality comes from Thor but I think its worth having a section in the Rails guide since it took some extra time to figure how to do this correctly.

Here is an example generator where I used command line arguments like this:
https://github.com/Shopify/shopify_app/blob/master/lib/generators/shopify_app/install/install_generator.rb

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