Add documentation for ~ opertor
There is an example.

![s1521726276 43](https://user-images.githubusercontent.com/119974/37774099-4aba906e-2dad-11e8-9683-3e9c572bbe14.png)
 Thanks
Documentation has example for contains - e.g. Facility.name.contains('tennis').
There should be example for reverse of this contain API / reverse of any than queries.
e.g. I want to search for a Facility for which the Facility.name does not contain "tennis".

See: https://stackoverflow.com/questions/25734566/operator-not-in-with-peewee 

Thanks
Deepak