Provide install / plugin instructions in README

[hlascelles]

This looks like a great tool.

Do you add it to your Gemfile? Can you avoid doing that by installing it as a plugin globally?

ie, can it be made that bundle plugin install bundler-dependencies work?

Thanks!

[pboling]

In other words... is this a standard Bundler Plugin? It looks like the answer is no, but would like to get the author's insight (not least because it is missing the plugins.rb file in the project root).

Reference: https://bundler.io/guides/bundler_plugins.html