Add aliyun oss driver document.

Hello, I want to add some documents about [docker-registry-driver-alioss](https://github.com/chris-jin/docker-registry-driver-alioss.git) to help China Docker user.
[Aliyun](http://www.aliyun.com/)(Alibaba Group) is the largest cloud computing company in China. [Aliyun OSS](http://www.aliyun.com/product/ess) have millions users in China. It is helpful for Chinese users who want to deploy private docker-registry in Aliyun OSS. Because of [GFW](http://en.wikipedia.org/wiki/Golden_Shield_Project), it is not easy for Chinese user download images from docker public registry, many docker users in China are deploy their docker-registry within the network of China. If the document about [docker-registry-driver-alioss](https://github.com/chris-jin/docker-registry-driver-alioss.git) added to docker-registry official document, it's very helpful for these users.