DOCUMENTATION: A vignette / example with a basic Shiny app using futures

Original subject: Please use my sample code in a vignette if helpful...

If you think it helpful please incorporate any and all ideas from some sample code I posted [here](http://stackoverflow.com/a/43188619/283973).

Review the sample code in the linked Stack Overflow answer and the repository's existing vignette structure. Add a vignette or example showing a basic Shiny app using futures, incorporating relevant ideas from the sample; the documentation should make the example runnable and explain the demonstrated usage.

Written by the indexing model from the issue text.