Added examples for rails deployers and couple additions

I think that it is a good place for example how to properly add capistrano extensions for rails, bundler and ruby version manager in first step of upgrade doc because right now Readme docs in those gems not in one style. Somewhere there is no `require: false` option, somewhere no `group: :development` option, in `capistrano` readme there is version locking, but in others – not everywhere. Also from one hand it's annoying to go somewhere else to look how to properly add extensions which before were included by default. But from the other hand it's helpful to look what settings there by default and how to set it properly - in this case better to add links to extensions' readme files.

I'm not sure about advice to set stage definition in `Capfile`. Is it a good practice to set any settings in `Capfile`?

I think that it'll be good to list in some step all current defaults to help users to remove excess settings from their deploy.rb. And add step about new `linked_files` and `linked_dirs` sections, because I think that it is widely used practice in cap v2 to write own tasks to symlink configs. What do you think?