Should we mention Ferrocene in the Appendix?

[chriskrycho]

While thinking about #3859, I started considering whether it makes sense to mention Ferrocene in that same spot. There is a reasonably large set of potential Rust users who find the 6-week support timeline a non-starter, and for good reason given the difficulty of getting new software toolchains approved for use; that is the whole raison d'être for Ferrocene! But it is also not an official Rust project, and is on offer for a fee from a private consultancy, so it might be odd in the official book. (To be extra clear, I am personally all in support of Ferrous Systems’ support model there!)

[carols10cents]

So this particular appendix is online-only, and I'm fine mentioning Ferrocene in that. I'd be a bit more hesitant to put it in print, mostly because I hope there will be more alternative compiler choices in the future.

Ideally, there'd be a page on rust-lang.org that links to any Rust compiler including Ferrocene, and then we can link to that page here as this list might get unwieldy and I don't think the book should be the definitive resource on the existence of alternative compiler toolchains, but we can start here for now.