Add custom shiny input project to README

README.md

 or read through the 
 [dev journal](https://github.com/gadenbuie/js4shiny-frappeCharts/blob/master/dev/dev.md)
 to explore the project and the changes made at each step of the process.
+
+---
+
+## Building Custom Shiny Inputs
+
+✨ Bonus! ✨
+
+This repo also contains a walkthough
+building a custom Shiny input: 
+a `<textarea>` input that measures typing speed.
+At the end of the project,
+we incorporate the `frappeCharts` widget
+we built in the [main project](https://github.com/gadenbuie/js4shiny-frappeCharts/blob/master/dev/dev.md).
+
+Walk through the 
+[commit log of the shiny-input branch](https://github.com/gadenbuie/js4shiny-frappeCharts/commits/shiny-input)
+or read through
+[the shiny-input dev journal](https://github.com/gadenbuie/js4shiny-frappeCharts/tree/shiny-input/inst/shiny-input-app#readme)
+to explore the project and the changes made at each step of the process.
+
+The final app is available to demo at https://gadenbuie.shinyapps.io/type-racer/