update build instructions (docker example)

[dratasich]

Easy build instructions for Android with docker.

The three lines of the build instructions were not sufficient for me (I'm cordova beginner). The enhanced instructions use docker and state detailed commands to get the build working.

Note I removed the platforms directory first. I didn't put this remark into the instructions, because I think this directory should be removed anyway.

I hope it is fine (not to long and clear) and somebody finds it useful.

[brvier]

Hum not a great fan of docker, but as many like it, i ll try your build instruction