docs: polish code style document for pouch

[allencloud]

Signed-off-by: Allen Sun allensun.shl@alibaba-inc.com

Ⅰ. Describe what this PR did

This pull request tries to polish the code style document of project Pouch. These code styles should be followed when contributors are collaborating in this open source project. We will encourage every contributor to read this code style document first, and then do the code change.

In this pull request, I put forward the following 10 code style rules which are additional:

• RULE001 - Add blank line between field's comments
• RULE002 - Add parameter name in interface definition
• RULE003 - Import Packages
• RULE004 - Variable declaration position
• RULE005 - Generation of action failure
• RULE006 - Return fast to ident less
• RULE007 - Lowercase log and error
• RULE008 - Nested errors
• RULE009 - Comment correctly
• RULE010 - Always remember DRY

In addition, I do not think we should include markdown lint rule in https://github.com/markdownlint/markdownlint/blob/master/docs/RULES.md#md010---hard-tabs .

I think both tabs and whitespaces are valid here. cc @HusterWan

Ⅱ. Does this pull request fix one issue?

none

Ⅲ. Describe how you did it

make code style document more readable and more specific.

Ⅳ. Describe how to verify it

none

Ⅴ. Special notes for reviews

none

[codecov-io]

Codecov Report

Merging #1283 into master will decrease coverage by 0.27%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1283      +/-   ##
==========================================
- Coverage   15.53%   15.26%   -0.28%     
==========================================
  Files         173      172       -1     
  Lines       10858    10698     -160     
==========================================
- Hits         1687     1633      -54     
+ Misses       9052     8945     -107     
- Partials      119      120       +1

Impacted Files	Coverage Δ
daemon/mgr/image_utils.go	0% <0%> (-40%)	⬇️
pkg/reference/def.go	33.33% <0%> (-11.12%)	⬇️
daemon/mgr/cri_utils.go	28.85% <0%> (-0.46%)	⬇️
cli/rm.go	0% <0%> (ø)	⬆️
apis/server/container_bridge.go	0% <0%> (ø)	⬆️
cli/image_list.go	0% <0%> (ø)	⬆️
cli/run.go	0% <0%> (ø)	⬆️
ctrd/container.go	0% <0%> (ø)	⬆️
daemon/mgr/volume.go	0% <0%> (ø)	⬆️
apis/server/volume_bridge.go	0% <0%> (ø)	⬆️
... and 14 more

[Letty5411]

LGTM