Rollup merge of rust-lang#29180 - SingingTree:readme_windows_build, r=steveklabnik

Adds the pacman git command to the high level Windows installation instructions. As the MSYS2 installation may not include Windows git on the path by default (based on the path settings of Windows users), I figured this could be a helpful inclusion.

Author: steveklabnik

README.md

@@ -72,6 +72,9 @@ Read ["Installing Rust"] from [The Book].
    $ pacman -S mingw-w64-i686-toolchain
    $ pacman -S mingw-w64-x86_64-toolchain
 
+   # Make git available in MSYS2 (if not already available on path)
+   $ pacman -S git
+
    $ pacman -S base-devel
    ```