Merge pull request #744 from brson/winreadme

Clarify how rustup toolchain installation works on Windows

README.md

@@ -461,11 +461,14 @@ Command | Description
 The primary installation method, as described at
 [www.rustup.rs](https://www.rustup.rs), differs by platform:
 
-* On Windows, download and run the [rustup-init.exe for the
-  `i686-pc-windows-gnu` target][setup]. Although this build of
-  `rustup` installs compilers targeting the GNU ABI by default,
-  compilers targeting the MSVC ABI can be installed with e.g. `rustup
-  install stable-msvc`.
+* On Windows, download and run the [rustup-init.exe built for
+  `i686-pc-windows-gnu` target][setup]. Despite being built against the GNU
+  toolchain, _the Windows build of rustup will install Rust for the MSVC
+  toolchain if it detects that MSVC is installed_. If you prefer to install GNU
+  toolchains or x86_64 toolchains by default this can be modified at install
+  time, either interactively or with the `--default-host` flag, or after
+  installation via `rustup set default-host`. In general, this is the build
+  of rustup one should install on Windows.
 * On Unix, run `curl https://sh.rustup.rs -sSf | sh` in your
   shell. This downloads and runs [`rustup-init.sh`], which in turn
   downloads and runs the correct version of the `rustup-init`