[docs] Add IO Warning to WSL section of Getting Started (#12384)

* Add IO Warning to WSL section of Getting Started

* FauxPark suggestion (thanks!)

Co-authored-by: Ryan <fauxpark@gmail.com>

Co-authored-by: Ryan <fauxpark@gmail.com>

docs/newbs_getting_started.md

@@ -70,6 +70,8 @@ Install the QMK CLI by running:
 
 ### ** Linux/WSL **
 
+?> **Note for WSL users**: By default, the installation process will clone the QMK repository into your WSL home directory, but if you have cloned manually, ensure that it is located inside the WSL instance instead of the Windows filesystem (ie. not in `/mnt`), as accessing it is currently [extremely slow](https://github.com/microsoft/WSL/issues/4197).
+
 #### Prerequisites
 
 You will need to install Git and Python. It's very likely that you already have both, but if not, one of the following commands should install them: