Add instructions of minimial PyTorch version

[zhiqwang]

Fixes #72

[codecov]

Codecov Report

Merging #74 (6e25f4a) into master (76f5a5d) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #74   +/-   ##
=======================================
  Coverage   80.35%   80.35%           
=======================================
  Files           8        8           
  Lines         682      682           
=======================================
  Hits          548      548           
  Misses        134      134           

Flag	Coverage Δ
unittests	80.35% <ø> (ø)

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