Add post install message for upgrading 2.x.x -> 3.x.x

[codealchemy]

Sample from running against a locally built copy of the last release candidate

$ gem install rails_admin-3.0.0.rc.gem --post-install-message

    ### Upgrading from 2.x.x to 3.x.x ###

    Due to introduction of Webpack/Webpacker support, some additional dependency and configuration will be needed.
    Running `bin/rails g rails_admin:install` will suggest required changes, based on the current setup of your app.

    For a complete list of changes, see https://github.com/railsadminteam/rails_admin/blob/master/CHANGELOG.md
  
Successfully installed rails_admin-3.0.0.rc
Parsing documentation for rails_admin-3.0.0.rc
Done installing documentation for rails_admin after 0 seconds
1 gem installed

This should help give some more guidance to users upgrading.

[mshibuya]

Great to have it 👍