Document setting RUSTFLAGS in the readme

[frewsxcv]

rust-fuzz/targets#1 (comment)

[Manishearth]

So you don't always want to set rustflags -- if the code you want fuzzed is in the same crate cargo rustc will work too. But usually you write a separate tiny fuzz script and link to the crate, in which case you need the actual crate to be instrumented.