Add qmk config user.overlay_dir step to README.md (qmk#23)

Author: arisudesu

README.md

@@ -7,6 +7,7 @@ This is a template repository which allows for an external set of QMK keymaps to
 1. Run the normal `qmk setup` procedure if you haven't already done so -- see [QMK Docs](https://docs.qmk.fm/#/newbs) for details.
 1. Fork this repository
 1. Clone your fork to your local machine
+1. Enable userspace in QMK config using `qmk config user.overlay_dir="$(realpath qmk_userspace)"`
 1. Add a new keymap for your board using `qmk new-keymap`
     * This will create a new keymap in the `keyboards` directory, in the same location that would normally be used in the main QMK repository. For example, if you wanted to add a keymap for the Planck, it will be created in `keyboards/planck/keymaps/<your keymap name>`
     * You can also create a new keymap using `qmk new-keymap -kb <your_keyboard> -km <your_keymap>`
@@ -55,4 +56,4 @@ This can also be used to control which fork is used, though only upstream `qmk_f
 
 1. (First time only) `git submodule add https://github.com/qmk/qmk_firmware.git`
 1. (To update) `git submodule update --init --recursive`
-1. Commit your changes to your userspace repository
+1. Commit your changes to your userspace repository