[Improvement] Add warnings of uninstalling mmdet and supplementary documents

[congee524]

fix #620

[codecov]

Codecov Report

Merging #624 (3a79acd) into master (c329052) will decrease coverage by 0.07%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master     #624      +/-   ##
==========================================
- Coverage   84.65%   84.57%   -0.08%     
==========================================
  Files         127      127              
  Lines        8945     8948       +3     
  Branches     1502     1503       +1     
==========================================
- Hits         7572     7568       -4     
- Misses       1017     1023       +6     
- Partials      356      357       +1     

Flag	Coverage Δ
unittests	84.57% <0.00%> (-0.07%)	⬇️

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Impacted Files	Coverage Δ
mmaction/models/builder.py	51.21% <0.00%> (-4.05%)	⬇️
mmaction/datasets/pipelines/augmentations.py	94.33% <0.00%> (-0.51%)	⬇️

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