Add some docs about exporting and importing snapshots using Aliyun object storage service(OSS)

Aliyun Object Storage Service(Aliyun OSS) is widely used, particularly popular among China’s cloud users, and it provides cloud object storage for a variety of use cases. Hadoop file system supports OSS since version 2.9.1. Now, we can export and import snapshots using Aliyun object storage service(OSS).