Docs: Plotly in Shiny

Three new files (generated with knitr) documenting an example of using Plotly in Shiny:
- plotly_in_shiny.Rmd;
- plotly_in_shiny.html; and,
- plotly_in_shiny.md

/cc @mkcor 
What kind of double quotes are these?  Just wondering if they are standard.  I think displaying `iris` as code here would work too.