Add mention of urllib3 support in readme.
Add mention of urllib3 support in readme.

poussik/vcrpy,gwillem/vcrpy,bcen/vcrpy,yarikoptic/vcrpy,mgeisler/vcrpy,kevin1024/vcrpy,kevin1024/vcrpy,IvanMalison/vcrpy,graingert/vcrpy,agriffis/vcrpy,ByteInternet/vcrpy,aclevy/vcrpy,poussik/vcrpy,graingert/vcrpy,ByteInternet/vcrpy