Update README with minimum required version of PyTorch

[fmassa]

Fix #1257

But I don't know what's needed to update PyPI website though.

[codecov-io]

Codecov Report

Merging #1272 into master will increase coverage by 0.03%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1272      +/-   ##
==========================================
+ Coverage   65.52%   65.56%   +0.03%     
==========================================
  Files          74       74              
  Lines        5784     5784              
  Branches      884      884              
==========================================
+ Hits         3790     3792       +2     
+ Misses       1728     1727       -1     
+ Partials      266      265       -1

Impacted Files	Coverage Δ
torchvision/transforms/transforms.py	80.55% <0%> (-0.4%)	⬇️
torchvision/transforms/functional.py	71.38% <0%> (+1.15%)	⬆️

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