Add a demo shiny app in the doc

It would be helpful to have a demo shiny app at: https://www.htmlwidgets.org/develop_intro.html#creating-your-own-widgets 

```r
library(shiny)
library(mywidget)
ui <- tagList(
    mywidgetOutput("x")
 )

server <- function(
  input,
  output,
  session
) {
  output$x <- renderMywidget({ 
    	mywidget("hello")
    })
}

shinyApp(ui, server)
```
