add introduction

[maxheld83]

intro.js might be a really cool way to get this done; it's also already used by some shiny packages: http://shiny.rstudio.com/tutorial/js-lesson1/

there's also:

• https://github.com/carlganz/rintrojs
• https://github.com/FrissAnalytics/shinyJsTutorials

[VerenaHeld]

• EnjoyHint is easy to use, but limited to one use case. The arrows and the dark background look nice, but this is not what we want. (Or maybe a temporary solution for Testlauf?).
• Trip.js Not so pretty, but more themes, no black background, easier to integrate in page. ...