Add mentions and links to honggfuzz-rs

[PaulGrandperrin]

Also, I think we should remove the mention that cargo-fuzz is the recommended fuzzer in this file: https://github.com/rust-fuzz/book/blob/master/src/cargo-fuzz.md. I think there is two reasons for this:

• honggfuzz-rs is now available. It has its own set of strenghts and weaknesses but I honestly think it is as relevant as cargo-fuzz.
• afs.rs has vastly improved in performance thanks to persistent fuzzing. I think it is now as relevant as the other two.

closes #10

[PaulGrandperrin]

That's really fine by me to say that cargo-fuzz is intended to eventually support all fuzzers, and users would then be able to judge how much this fact is important to them. What I think is that there is plenty of other factors to choose a fuzzer instead of another, and planned compatibility is only one of them. The three fuzzers we have are all very interesting because they have a different set of strengths and weaknesses and I don't think we can now (in 2018) simply recommend one over the others. Maybe we will be able to do that in the future though.