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### _Why are the changes needed?_
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This pr tries to improve some contents in README.md.

### _How was this patch tested?_
Only doc, no need to test.

Closes #523 from wzhfy/improve_doc.

Closes #523

efd64b45 [Zhenhua Wang] improve
Authored-by: Zhenhua Wang <wzh_zju@163.com>
Signed-off-by: Cheng Pan <379377944@qq.com>