Rollup merge of #60816 - scottmcm:vcpp-download-link, r=alexcrichton
Rollup merge of #60816 - scottmcm:vcpp-download-link, r=alexcrichton

README.md: Mention MSVC 2017+, not 2013(!)

LLVM will soon require 2017+ (and our in-tree version just rejected the version of 2015 I was using), so update the mention and provide a link.

Ref: https://llvm.org/docs/GettingStarted.html#host-c-toolchain-both-compiler-and-standard-library

graydon/rust,graydon/rust,graydon/rust,aidancully/rust,graydon/rust,aidancully/rust,aidancully/rust,aidancully/rust,graydon/rust,aidancully/rust,aidancully/rust,graydon/rust