Update prerequisites, with an eye towards windows (#863)

* Update prerequisites, with an eye towards windows

* Address review comments

- Say version of MSVC needed
- Link to more details windows section
- Fix grammar

Author: jyn514

src/building/prerequisites.md

@@ -4,20 +4,28 @@
 
 Before building the compiler, you need the following things installed:
 
-* `g++` 5.1 or later or `clang++` 3.5 or later
-* `python` 3 or 2.7
-* GNU `make` 3.81 or later
-* `cmake` 3.4.3 or later
+* `python` 3 or 2.7 (under the name `python`; `python2` or `python3` will not work)
 * `curl`
 * `git`
 * `ssl` which comes in `libssl-dev` or `openssl-devel`
 * `pkg-config` if you are compiling on Linux and targeting Linux
 
-Additionally, if you want to build `rustc` with your system's LLVM, you will
-need `llvm-config`. See [this section for more info][sysllvm].
+If building LLVM from source (the default), you'll need additional tools:
+
+* `g++` 5.1 or later, `clang++` 3.5 or later, or MSVC 2017 or later.
+* `ninja`, or GNU `make` 3.81 or later (ninja is recommended, especially on Windows)
+* `cmake` 3.4.3 or later
+
+Otherwise, you'll need LLVM installed and `llvm-config` in your path.
+See [this section for more info][sysllvm].
 
 [sysllvm]: ./suggested.md#building-with-system-llvm
 
+### Windows
+
+For more information about building on Windows,
+see [the Rust README](https://github.com/rust-lang/rust#msvc).
+
 ## Hardware
 
 These are not so much requirements as _recommendations_: