Updates to main readme.
Add much more info to usage article.
Move simulator building to simulation's readme.
Improve assembly article.
Many fixes.
Updates in diagrams.
Arm and disarm with gestures only: left stick right/down for arming, left/down for disarming.
Remove arming switch as it complicates arming gestures logic.
Remove MAV_CTRL_SCALE parameter as it complicates arming gestures logic, advise to decrease TILT_MAX when controlling with a smartphone.
Put some minimal thrust to motors to indicate armed state.
Rename build article to usage article, add flight instructions.
