Update feature documentation

docs/feature_velocikey.md

@@ -3,7 +3,17 @@
 Velocikey is a feature that lets you control the speed of lighting effects (like the Rainbow Swirl effect) with the speed of your typing. The faster you type, the faster the lights will go!
 
 ## Usage
-For Velocikey to take effect, you need to enable it with the VLK_TOG keycode, which toggles it on and off.
+For Velocikey to take effect, there are two steps. First, when compiling your keyboard, you'll need to set `VELOCIKEY_ENABLE=yes` in `rules.mk`, e.g.:
+
+```
+BOOTMAGIC_ENABLE = no
+MOUSEKEY_ENABLE = no
+STENO_ENABLE = no
+EXTRAKEY_ENABLE = yes
+VELOCIKEY_ENABLE = yes
+```
+
+Then, while using your keyboard, you need to also turn it on with the VLK_TOG keycode, which toggles the feature on and off.
 
 The following light effects will all be controlled by Velocikey when it is enabled:
  - RGB Breathing