Add more advanced usage docs
Add more advanced usage docs

* Document how to use custom ssl certs and client certs.
* Document how to customize pool behavior.

Disassem/urllib3,Disassem/urllib3,urllib3/urllib3,sigmavirus24/urllib3,Lukasa/urllib3,haikuginger/urllib3,urllib3/urllib3,Lukasa/urllib3,sigmavirus24/urllib3,haikuginger/urllib3