Update build instructions for app.
Update build instructions for app.

As the lazybones app is no longer a simple Groovy script, I have updated the
build instructions in the README. To be honest, the build is still very simple
but now developers will at least know the entry point for the code, making it
easier to contribute.

danielgrycman/lazybones,aaronzirbes/lazybones,ctran/lazybones,danielgrycman/lazybones,pledbrook/lazybones,pledbrook/lazybones-kotlin,pledbrook/lazybones,pledbrook/lazybones-kotlin,ctran/lazybones