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).
I’ll ask you to not reference Vagrant directly, since I’ll change the example later today (as per suggestion in #4992). I’ll do it with Unity, which is a big download (that I’m currently updating). Not referring to any specific name but solely as the “example above” should also cover us in the future.