doc: Refactor

[gaocegege]

The README is a liitle long and I think there are something that the users do not need know. Thus I split it into different parts in docs.

PTAL

Signed-off-by: Ce Gao gaoce@caicloud.io

[gaocegege]

/assign @YujiOshima

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[YujiOshima]

/lgtm

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