[Doc]: add a short document for TensorRT plugins in mmcv

[RunningLeon]

This PR adds a short document for TensorRT plugins in mmcv.

[codecov]

Codecov Report

Merging #797 (81fc196) into master (8e3a801) will increase coverage by 0.12%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #797      +/-   ##
==========================================
+ Coverage   61.85%   61.97%   +0.12%     
==========================================
  Files         144      144              
  Lines        8467     8468       +1     
  Branches     1520     1520              
==========================================
+ Hits         5237     5248      +11     
+ Misses       2962     2953       -9     
+ Partials      268      267       -1     

Flag	Coverage Δ
unittests	61.97% <ø> (+0.12%)	⬆️

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Impacted Files	Coverage Δ
mmcv/fileio/handlers/yaml_handler.py	87.50% <0.00%> (-12.50%)	⬇️
mmcv/tensorrt/tensorrt_utils.py	1.03% <0.00%> (+1.03%)	⬆️
mmcv/image/io.py	90.75% <0.00%> (+1.77%)	⬆️
mmcv/tensorrt/init_plugins.py	43.75% <0.00%> (+43.75%)	⬆️
mmcv/tensorrt/__init__.py	50.00% <0.00%> (+50.00%)	⬆️

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