Disambiguate safety language

> IMPORTANT: By using the Caskfiles in this repository, you are tapping into **potentially untrusted software**!

My inner pedant suggests that the software tapped here would be better described as “untrusted” (not *potentially* so; simply *untrusted*) or “potentially unsafe”.

> While we try our best to audit Caskfiles and ensure their safety and operation, […]

This statement is not currently problematic, but it might become soon.

A user who is unfamiliar with Cask might be mislead to think that we audit the casked software rather than the cask definition files. This is almost true as of writing, because all unofficial casks but one are maintained by core contributors @alebcay and @rolandwalker; we trust them and can claim that their builds are audited. (There would be a more fundamental issue here, but introducing it would be off-topic.)

However, were we to accept external contributions, claiming that we audit what is casked would be both false and unfeasible.

The issue could be mitigated by establishing restrictive admissibility guidelines for `homebrew-unofficial`: at a minimum, source should be mandated. Nonetheless, improving the language we use in `README` would remain desirable.