[Doc] Update CSN models' README & metafile

[hukkai]

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Motivation

Please describe the motivation of this PR and the goal you want to achieve through this PR.

Modification

Please briefly describe what modification is made in this PR.

BC-breaking (Optional)

Does the modification introduces changes that break the back-compatibility of this repo?
If so, please describe how it breaks the compatibility and how users should modify their codes to keep compatibility with this PR.

Use cases (Optional)

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

Checklist

1. Pre-commit or other linting tools should be used to fix the potential lint issues.
2. The modification should be covered by complete unit tests. If not, please add more unit tests to ensure the correctness.
3. If the modification has potential influence on downstream projects, this PR should be tested with downstream projects, like MMDet or MMCls.
4. The documentation should be modified accordingly, like docstring or example tutorials.

[codecov]

Codecov Report

Base: 75.35% // Head: 75.35% // No change to project coverage 👍

Coverage data is based on head (f57915e) compared to base (37ce660).
Patch has no changes to coverable lines.

Additional details and impacted files

@@           Coverage Diff            @@
##           dev-1.x    #2121   +/-   ##
========================================
  Coverage    75.35%   75.35%           
========================================
  Files          143      143           
  Lines        10856    10856           
  Branches      1780     1780           
========================================
  Hits          8180     8180           
  Misses        2269     2269           
  Partials       407      407           

Flag	Coverage Δ
unittests	75.35% <ø> (ø)

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