Auto merge of #28042 - Jake-Shadle:master, r=steveklabnik

Encountered an issue with `pacman` while 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)).

I thought it would be good to update the README so people don't get frustrated. 😃 

r? @steveklabnik

Author: bors

README.md

@@ -65,6 +65,9 @@ Read ["Installing Rust"] from [The Book].
    tools.
 
    ```sh
+   # Update package mirrors (may be needed if you have a fresh install of MSYS2)
+   $ pacman -Sy pacman-mirrors
+   
    # Choose one based on platform:
    $ pacman -S mingw-w64-i686-toolchain
    $ pacman -S mingw-w64-x86_64-toolchain