[Documentation] Add Chinese Tutorial

[wangruohui]

No description provided.

[dreamerlin]

1. Change the name to mmaction2_tutorial_zh-CN.ipynb
2. Add the link to README_zh-CN.md like https://github.com/open-mmlab/mmaction2/pull/940/files

[codecov]

Codecov Report

Merging #941 (c686a4e) into master (e91ee61) will increase coverage by 0.03%.
The diff coverage is n/a.

❗ Current head c686a4e differs from pull request most recent head 0be78eb. Consider uploading reports for the commit 0be78eb to get more accurate results

@@            Coverage Diff             @@
##           master     #941      +/-   ##
==========================================
+ Coverage   83.69%   83.72%   +0.03%     
==========================================
  Files         132      132              
  Lines        9983     9983              
  Branches     1722     1722              
==========================================
+ Hits         8355     8358       +3     
+ Misses       1211     1210       -1     
+ Partials      417      415       -2     

Flag	Coverage Δ
unittests	83.72% <ø> (+0.03%)	⬆️

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Impacted Files	Coverage Δ
mmaction/datasets/pipelines/augmentations.py	92.98% <0.00%> (+0.34%)	⬆️

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