[docs] Missed Usage and System Design Documentation in README

[gaocegege]

Issue Description

Missed Usage and System Design Documentation in README

Describe what happened:

https://github.com/alibaba/pouch/blame/master/README.md#L15

Describe what you expected to happen:

Add the corresponding documents.

How to reproduce it (as minimally and precisely as possible):

Anything else we need to know?:

Environment:

[pouchrobot]

Thanks for your contribution. 🍻 @gaocegege
While we thought ISSUE DESCRIPTION could be more specific, longer than 100 chars.
Here is a template at https://github.com/alibaba/pouch/blob/master/.github/ISSUE_TEMPLATE.md
Please edit this issue description instead of opening a new one.
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[gaocegege]

PTAL @pouchrobot

[allencloud]

Thanks for your issue. @gaocegege
Yeah, what you mentioned is totally right. Actually this is covered in pr #108.

[allencloud]

#108 has been merged.
I think we could close this issue. What do you think of that? @gaocegege
And feel free to tell us if you meet any problem. Thanks a lot.