Document getting started without sanitizers.

[Eh2406]

Apparently cargo-fuzz supports being used without sanitizers. This is mentioned on Reddit and in #158. After stumbling across this information I tried looking at the readme which claims:

Note: libFuzzer needs LLVM sanitizer support, ...

And the book which claims:

libFuzzer needs LLVM sanitizer support, ...

My first interpretation was that the post and issue were out of date, and sanitizer support was added back in. But I'm not sure. Is the documentation just out of date?

Can we clarify the introductory documentation about whether sanitizers are needed? At a minimum by changing those sentences. Even better would be adding instructions on how to get started if you explicitly do not want to use a sanitizer.

[fitzgen]

libfuzzer needs sancov to track coverage but doesn't need asan or anything like that.