Mention Windows (Vcpkg) build in README.md
Mention Windows (Vcpkg) build in README.md

Summary: Closes https://github.com/facebook/folly/pull/697

Reviewed By: yfeldblum

Differential Revision: D6144274

Pulled By: Orvid

fbshipit-source-id: a79a2e36e8fcf271925e97ece2a6adbb3c074216

facebook/folly,rklabs/folly,Orvid/folly,facebook/folly,rklabs/folly,Orvid/folly,Orvid/folly,Orvid/folly,rklabs/folly,rklabs/folly,facebook/folly,rklabs/folly,facebook/folly,facebook/folly,Orvid/folly