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

LLVM will soon require 2017+ [1] (and our in-tree version just rejected the version of 2015 I was using), so update the mention and provide a link.

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

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