Doc for explaining how the stork work as a whole

[peter-wangxu]

Is this a BUG REPORT or FEATURE REQUEST?:
DOC request
What happened:
The readme give me an overview for functionality instead of technical implementation
What you expected to happen:
More details on implemetation, eg: I want to know how a PV was provisioned
How to reproduce it (as minimally and precisely as possible):

Anything else we need to know?:

Environment:

• Kubernetes version (use kubectl version):
• Cloud provider or hardware configuration:
• OS (e.g. from /etc/os-release):
• Kernel (e.g. uname -a):
• Install tools:
• Others: