Updated README to account for changes in MSYS2

One of the newest versions of MSYS2 now only has one .cmd file which replaces the old bat files. It has to be used to launch the mingw32/64 shell.

Author: seventh-chord

README.md

@@ -90,6 +90,8 @@ installing from pacman should be just fine.
 
 3. Run `mingw32_shell.bat` or `mingw64_shell.bat` from wherever you installed
    MSYS2 (i.e. `C:\msys`), depending on whether you want 32-bit or 64-bit Rust.
+   (As of the latest version of MSYS2 you have to run `msys2_shell.cmd -mingw32`
+   or `msys2_shell.cmd -mingw64` from the command line instead)
 
 4. Navigate to Rust's source code, configure and build it: