Update INSTALL.md instructions for MinGW

INSTALL.md

@@ -145,10 +145,9 @@ toolchain.
 
 1. Download the latest [MSYS2 installer][msys2] and go through the installer.
 
-2. Run `mingw32_shell.bat` or `mingw64_shell.bat` from the MSYS2 installation
-   directory (e.g. `C:\msys64`), 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.)
+2. Start a MINGW64 or MINGW32 shell (depending on whether you want 32-bit
+   or 64-bit Rust) either from your start menu, or by running `mingw64.exe`
+   or `mingw32.exe` from your MSYS2 installation directory (e.g. `C:\msys64`).
 
 3. From this terminal, install the required tools:
 
@@ -157,8 +156,7 @@ toolchain.
    pacman -Sy pacman-mirrors
 
    # Install build tools needed for Rust. If you're building a 32-bit compiler,
-   # then replace "x86_64" below with "i686". If you've already got Git, Python,
-   # or CMake installed and in PATH you can remove them from this list.
+   # then replace "x86_64" below with "i686". 
    # Note that it is important that you do **not** use the 'python2', 'cmake',
    # and 'ninja' packages from the 'msys2' subsystem.
    # The build has historically been known to fail with these packages.
@@ -178,6 +176,18 @@ toolchain.
    python x.py setup user && python x.py build && python x.py install
    ```
 
+If you want to use the native versions of Git, Python, or CMake you can remove
+them from the above pacman command and install them from another source. Make
+sure that they're in your Windows PATH, and edit the relevant `mingw[32|64].ini`
+file in your MSYS2 installation directory by uncommenting the line
+`MSYS2_PATH_TYPE=inherit` to include them in your MSYS2 PATH.
+
+Using Windows native Python can be helpful if you get errors when building LLVM.
+You may also want to use Git for Windows, as it is often *much* faster. Turning
+off real-time protection in the Windows Virus & Threat protections settings can
+also help with long run times (although note that it will automatically turn
+itself back on after some time).
+
 ### MSVC
 
 MSVC builds of Rust additionally require an installation of Visual Studio 2017