Update README.md

Encountered an issue going through the guide for installing the `mingw` toolchain on Windows with msys2, after some googling I found the [solution](msys2/MSYS2-packages#163 (comment)) and thought it would be good to update the README so people don't get frustrated. :)

Author: Jake Shadle

README.md

@@ -72,6 +72,22 @@ Read ["Installing Rust"] from [The Book].
    $ pacman -S base-devel
    ```
 
+   > ***Note:*** If the package install fails during retrieval, you may need to update
+   > `pacman`'s mirrors before attempting to install the `mingw64` toolchain. An example
+   > of the symptom of this problem would be...
+  
+   > ```sh
+   > error: failed retrieving file 'mingw-w64-x86_64-libiconv-1.14-4-any.pkg.tar.xz'
+   > from downloads.sourceforge.net : The requested URL returned error: 404
+   > ```
+  
+   > To update the mirrors for pacman, simply run the following commands in your
+   > MSYS2 terminal and retry installing the `mingw64` toolchain.
+  
+   > ```sh
+   > $ pacman -Sy&&pacman -S pacman-mirrors
+   > ```
+
 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.