Mention that the Isolation property listed in the readme is not a security property

[Ekleog]

Hello,

Following discussions on IRC, I've been pointed to this crate. It looks great!

That said, there is a thing that is listed in the readme, and, I think, has been misinterpreted by some: the Isolation property does not protect against a malicious proc macro, only against a buggy proc macro.

It doesn't protect against an actively malicious proc macro, because just wrapping the malicious code in quote!{} and returning it alongside the token streams would be enough to bypass any isolation properties of wasm -- except if the built code is never run locally, even just for cargo test.

As such, I think it may be helpful to just add a sentence to the readme stating that this isolation property does not protect against actively malicious proc macros?