CONTRIBUTING.md: Adding unversioned Cask info

The docs seemed to suggest that if versioned downloads were available,
to prefer them over unversioned ones. Since that's incorrect presently,
I attempted to correct that (see #6126 for more details).
But that's specifically what screwed me up. Making no explicit recommendation would lead me and other potential newbie contributors to think `version 'latest'`/`sha256 :no_check` is an option for the sake of speed and/or laziness and to be avoided whenever possible.

Right now, if there are both versioned and unversioned downloads available, the policy as I've now been illuminated on is to prefer the unversioned download. The fact that this will change in the future is irrelevant, IMO, since a contributor isn't going to take the time to delve into all the tickets in addition to reading CONTRIBUTING.md. (And if they do, they may still come to the wrong conclusion as I did.)

Feel free to open an issue against me, now, to fix this line once the future arrives.