Some clarifications added

I added a few tips to help noobs get started with registering helper functions and validations.  Prior to my changes, the docs did not fully demonstrate where to add these rules, so the user was left guessing and googling for possible solutions.
Good call. It would be good to make mention of the standard of *not* using that to your advantage though, so that a new dev looking at the codebase can easily know where the _s are in the column names just based on the capitalization.