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 
Awesome, thanks @pedrodz for including that. Very smoothly referenced.