add pointer to windows FAQ in contributing.md (#17450)
Summary:
" ProTip! Great commit summaries contain fewer than 50 characters. Place extra information in the extended description."
lol
Pull Request resolved: https://github.com/pytorch/pytorch/pull/17450
Differential Revision: D14206500
Pulled By: suo
fbshipit-source-id: af7ffe299f8c8f04fa8e720847a1f6d576ebafc1
