Adding NKRO virtual dip-switch, using existing bit in keymap byte.

This takes the last, reserved bit there, but doesn't necessitate
a revision to the magic number because it doesn't alter byte order.
Add reference to NKRO virtual dip-switch to documentation.

README.md

@@ -151,6 +151,7 @@ To avoid configuring accidentally additive salt key `KC_SPACE` also needs to be
 - Disable Gui(`Left Gui`)
 - Swap Grave and Escape(`Grave`)
 - Swap BackSlash and BackSpace(`Back Slash`)
+- Enable NKRO on boot(`N`)
 
 #### Default Layer
 - Set Default Layer to 0(`0`)