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 
Totally splitting hairs here, but I think the R community may like strict terminology: 'package' here, instead of 'library'.  The library is supposed to be directory where the package gets installed.  But really this wording is not shocking at all!