Document use of Python launcher for Windows

The Python Launcher for Windows (PEP 397) is a far superior method of launching Python scripts on a Windows system.  

This adds documentation of the method of using a py.ini settings file to launch kivy programs using their supplied python version and environment (i.e. kivy.bat), while leaving operation of other Python scripts unaltered, by inclusion of a hash-bang (#!) line.

Unix-like systems can also use this method to enable double-click operation. Instructions for that are included.

The kivy demo scripts are modified to use this start method.  They will be no change to their operation when started in the traditional manner.