Add link to article about Unicode Hex Input in readme

For me it wasn't clear what is `Unicode Hex Input`, that article
  clarifies it (at least for me).

readme.md

@@ -906,7 +906,7 @@ In `quantum/keymap_extras/`, you'll see various language files - these work the
 
 ## Unicode support
 
-You can currently send 4 hex digits with your OS-specific modifier key (RALT for OSX with the "Unicode Hex Input" layout) - this is currently limited to supporting one OS at a time, and requires a recompile for switching. 8 digit hex codes are being worked on. The keycode function is `UC(n)`, where *n* is a 4 digit hexidecimal. Enable from the Makefile.
+You can currently send 4 hex digits with your OS-specific modifier key (RALT for OSX with the "Unicode Hex Input" layout, see [this article](http://www.poynton.com/notes/misc/mac-unicode-hex-input.html) to learn more) - this is currently limited to supporting one OS at a time, and requires a recompile for switching. 8 digit hex codes are being worked on. The keycode function is `UC(n)`, where *n* is a 4 digit hexidecimal. Enable from the Makefile.
 
 ## Backlight Breathing