update readme for rails 4 asset precompilation

When running `bundle exec rake assets:precompile:nondigest` the user will get an unknown rake task error. This is because `assets:precompile:nondigest` was removed in Rails 4. You are now recommended to use `rake assets:precompile`.

`rake assets:precompile` will digest your assets, but it will not compile them for the development environment. Therefore, we add `RAILS_ENV=development` to the front of the script. To ensure performance is maintained, we also want to turn off `config.assets.debug`.