Add info about disabling warnings for end users
Add info about disabling warnings for end users

For example, you can use `PYTHONWARNINGS` to disable warnings without
modifying code (when using Python >= 2.7).

Relates to:

- https://github.com/pypa/pip/pull/3109
- https://github.com/pypa/pip/issues/2681

haikuginger/urllib3,matejcik/urllib3,boyxuper/urllib3,silveringsea/urllib3,tutumcloud/urllib3,sigmavirus24/urllib3,Disassem/urllib3,haikuginger/urllib3,mikelambert/urllib3,denim2x/urllib3,denim2x/urllib3,sigmavirus24/urllib3,tutumcloud/urllib3,urllib3/urllib3,Lukasa/urllib3,matejcik/urllib3,sileht/urllib3,urllib3/urllib3,boyxuper/urllib3,silveringsea/urllib3,Lukasa/urllib3,sileht/urllib3,Disassem/urllib3,mikelambert/urllib3