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

Author: scottmcm

README.md

@@ -128,9 +128,15 @@ build.
 #### MSVC
 [windows-msvc]: #windows-msvc
 
-MSVC builds of Rust additionally require an installation of Visual Studio 2013
-(or later) so `rustc` can use its linker. Make sure to check the “C++ tools”
-option.
+MSVC builds of Rust additionally require an installation of Visual Studio 2017
+(or later) so `rustc` can use its linker.  The simplest way is to get the
+[Visual Studio Build Tools] and check the “C++ build tools” workload.
+
+[Visual Studio Build Tools]: https://visualstudio.microsoft.com/downloads/#build-tools-for-visual-studio-2019
+
+At last check (cmake 3.14.3 and msvc 16.0.3) using the 2019 tools fails to
+build the in-tree LLVM build with a CMake error, so use 2017 instead by
+including the “MSVC v141 – VS 2017 C++ x64/x86 build tools (v14.16)” component.
 
 With these dependencies installed, you can build the compiler in a `cmd.exe`
 shell with: