Minor README update for root path routes when overriding PagesController

A very minor update to the Overriding section reminding users to update their root route when overriding HighVoltage::PagesController.

This bit me for a few moments until I realized I was "Doing It Wrong™"; this change may save some hapless user 15 seconds or more of banging their head on their keyboard.

Cheers!
-Craig