📝 Document steps re: Karabiner kernel extension

Author: jasonrudolph

README.md

@@ -181,6 +181,10 @@ This setup is honed and tested with the following dependencies.
 
 2. Enable accessibility to allow Hammerspoon to do its thing [[screenshot]](screenshots/accessibility-permissions-for-hammerspoon.png)
 
+3. On macOS High Sierra or later, you'll be [prompted to allow Karabiner-Elements to load its kernel extension](https://pqrs.org/osx/karabiner/document.html#usage). Follow the prompts to upgrade your life:
+    1. Click "Open System Preferences" [[screenshot]](https://github.com/jasonrudolph/keyboard/blob/v5.0.0/screenshots/karabiner-elements-system-extension-prompt-1.png)
+    1. Click "Allow" [[screenshot]](https://github.com/jasonrudolph/keyboard/blob/v5.0.0/screenshots/karabiner-elements-system-extension-prompt-2.png)
+
 ## TODO
 
 - Add [#13](https://github.com/jasonrudolph/keyboard/pull/13) to [features](#features):