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 
Yes but let's not use your personal account! ;)
I have created an ad-hoc account: `(username="r_user_guide", key="mw5isa4yqp")`