Add comments about requirements

[zhiqwang]

Following the upstream YOLOv5: ultralytics/yolov5#2604 (comment)

Cython should be a dependency of the remaining packages in requirements.txt, so should be installed anyway even if not a direct requirement.

[codecov]

Codecov Report

Merging #96 (2324e73) into master (999fcb2) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #96   +/-   ##
=======================================
  Coverage   83.39%   83.39%           
=======================================
  Files           8        8           
  Lines         777      777           
=======================================
  Hits          648      648           
  Misses        129      129           

Flag	Coverage Δ
unittests	83.39% <ø> (ø)

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