Update Ruby action doc on how to use Gem file to add dependencies

The following medium post explains how to include 3rd party dependencies using Gem file and bundler.
https://medium.com/openwhisk/ruby-goes-serverless-apache-openwhisk-adds-native-support-for-ruby-7134faee14e

It would be awesome if someone would add a section to the [Ruby docs](https://github.com/apache/incubator-openwhisk/blob/master/docs/actions-ruby.md) on how to create a zip ruby action that includes dependencies using a Gem file and bundler.

Start with docs/actions-ruby.md and read the linked Medium post for the Ruby Gemfile and Bundler workflow. Add a section explaining how to create a zip Ruby action that includes third-party dependencies using a Gemfile and Bundler; the documentation should cover the requested workflow clearly.

Written by the indexing model from the issue text.