[Docs] Add PR document

[MeowZheng]

Motivation

• This document is to help users to contribute their work to OpenMMLab repos.

Modification

• Add pr.md in docs/community and its Chinese version in docs_zh_CN/community.

[codecov]

Codecov Report

Merging #1209 (ff99455) into master (96c4b70) will increase coverage by 0.29%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1209      +/-   ##
==========================================
+ Coverage   67.97%   68.27%   +0.29%     
==========================================
  Files         159      160       +1     
  Lines       10434    10599     +165     
  Branches     1899     1937      +38     
==========================================
+ Hits         7093     7236     +143     
- Misses       2970     2979       +9     
- Partials      371      384      +13     

Flag	Coverage Δ
unittests	68.27% <ø> (+0.29%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
mmcv/version.py	82.35% <0.00%> (-9.96%)	⬇️
mmcv/cnn/utils/weight_init.py	85.18% <0.00%> (-6.16%)	⬇️
mmcv/utils/logging.py	95.23% <0.00%> (-4.77%)	⬇️
mmcv/runner/hooks/logger/tensorboard.py	34.37% <0.00%> (-1.99%)	⬇️
mmcv/parallel/distributed.py	20.83% <0.00%> (-1.62%)	⬇️
mmcv/parallel/distributed_deprecated.py	66.66% <0.00%> (-0.73%)	⬇️
mmcv/ops/upfirdn2d.py	14.58% <0.00%> (-0.72%)	⬇️
mmcv/runner/hooks/optimizer.py	16.66% <0.00%> (-0.63%)	⬇️
mmcv/cnn/bricks/activation.py	87.50% <0.00%> (-0.50%)	⬇️
mmcv/runner/dist_utils.py	50.00% <0.00%> (-0.50%)	⬇️
... and 14 more

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[zhouzaida]

please fix the lint

[ZwwWayne]

Please make 3.png smaller

[zhouzaida]

LGTM