Add coffee-rails update information to rails guides

[iarobinson]

Summary

When upgrading from Rails 5.2 to 6.0, I found the following error:

DEPRECATION WARNING: Single arity template handlers are deprecated. Template handlers must
now accept two parameters, the view object and the source for the view object.
Change:
  >> Coffee::Rails::TemplateHandler.call(template)
To:
  >> Coffee::Rails::TemplateHandler.call(template, source)
 (called from <main> at /Users/MediocreManta/Desktop/freedom_podcasting/Rakefile:6)

The fix to this is to update your version of coffee-rails.

This PR adds instructions for upgrading coffee-rails in the instructions for migrating from Rails 5.2 to 6.0.

[kaspth]

In this case, our update guides assume knowledge of bundle and how to update a particular gem, so I'm not really keen on adding this.

I'd tentatively consider merging a deprecation warning fix since the deprecation warning wrongly points to your app's Rakefile and the coffee-rails. But for now the warning does mention Coffee::Rails, so I'll close this particular PR. Thanks!