Please add demo instructions to README

[maltfield]

Please add some instructions for the user to try out this module.

This repo has screenshots of the example, but the README makes no hint as to how that demo can be accessed. After some time, I had to dig through the code to figure out that it was main function of the __init_.py file.

Please add a "Demo" section to the main README.md file of this repo that says something to the effect of:

You can test the functionality of garden.navigationdrawer by executing the demo as follows:

git clone https://github.com/kivy-garden/garden.navigationdrawer.git
python3 garden.navigationdrawer/__init__.py

This should open the following interactive demo:
(put the screenshot image here)