Add a new "Basic use" section so people don't get overhauled with build instructions. They can read about the more advanced commands once they are able to use the basic ones. While at it, update the instructions to use the waf wrapper.
The necessity for that was pointed out in the PR #3841 ("waf: readme: update information regarding groups" - https://github.com/ArduPilot/ardupilot/pull/3841).
That makes it easier for new developers to find build instructions