[Docs] Translate contributing documentation

[zhouzaida]

Thanks for your contribution and we appreciate it a lot. The following instructions would make your pull request more healthy and more easily get feedback. If you do not understand some items, don't worry, just make the pull request and seek help from maintainers.

Motivation

Translate docs/communtiy/constributing.md

Modification

Add docs_zh_CN/communtiy/constributing.md

BC-breaking (Optional)

No

Use cases (Optional)

If this PR introduces a new feature, it is better to list some use cases here, and update the documentation.

Checklist

Before PR:

• I have read and followed the workflow indicated in the CONTRIBUTING.md to create this PR.
• Pre-commit or linting tools indicated in CONTRIBUTING.md are used to fix the potential lint issues.
• Bug fixes are covered by unit tests, the case that causes the bug should be added in the unit tests.
• New functionalities are covered by complete unit tests. If not, please add more unit test to ensure the correctness.
• The documentation has been modified accordingly, including docstring or example tutorials.

After PR:

• If the modification has potential influence on downstream or other related projects, this PR should be tested with some of those projects, like MMDet or MMCls.
• CLA has been signed and all committers have signed the CLA in this PR.

[codecov]

Codecov Report

Merging #1351 (fe8da99) into master (f3dfc41) will increase coverage by 0.31%.
The diff coverage is n/a.

❗ Current head fe8da99 differs from pull request most recent head 6c248ac. Consider uploading reports for the commit 6c248ac to get more accurate results

@@            Coverage Diff             @@
##           master    #1351      +/-   ##
==========================================
+ Coverage   68.82%   69.14%   +0.31%     
==========================================
  Files         163      162       -1     
  Lines       10811    10746      -65     
  Branches     1992     1978      -14     
==========================================
- Hits         7441     7430      -11     
+ Misses       2979     2927      -52     
+ Partials      391      389       -2     

Flag	Coverage Δ
unittests	69.14% <ø> (+0.31%)	⬆️

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

Impacted Files	Coverage Δ
mmcv/ops/__init__.py	100.00% <0.00%> (ø)
mmcv/utils/ext_loader.py	35.89% <0.00%> (ø)
mmcv/ops/ball_query.py
mmcv/ops/sync_bn.py	19.64% <0.00%> (+4.14%)	⬆️
mmcv/ops/contour_expand.py	100.00% <0.00%> (+25.00%)	⬆️
mmcv/ops/pixel_group.py	100.00% <0.00%> (+27.27%)	⬆️

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