Add mirror usage instructions.

Hi, being in China www.pypi-mirrors.org has been very useful, thanks for that! Standard mirror is very (very!) slow here. I would have saved some time if the page showed how to use the mirrors. So here is a commit with mirror usage instructions, feel free to reject/modify it as you will.

Screenshot:
![image](https://f.cloud.github.com/assets/919180/1995161/394b3ad2-84ff-11e3-92d7-0b68b4ba83c0.png)