Improve README

[ganler]

• Tested by grammarly
• Refine some layout stuff to make it more structural and compact
• Refine Python Training code samples to make it shorter @Gyx-One Please have a look here.

Also cc @luomai

[ganler]

Hey guys, pre-view is available here: https://github.com/tensorlayer/hyperpose/tree/doc-v2.2.0