Building afl.rs instructions

[Ralph Bisschops]

I was testing out this tool, but missed some build instructions.
Here are some example build instructions that worked for me:

## Install

Requirements:
- LLVM
- clang
Debian/Ubuntu based:
```bash
sudo apt-get install clang llvm
```

Install afl.rs
```bash
cargo +nightly install afl
```

Maybe add something like this to the README.md so other people can get started quicker.
Or add them to the book, because there was not much there. And I had gcc installed just did not build for some reason. (maybe because llvm was not installed, installed clang first because of the error I got)

[petr-tik]

Thanks for writing these down. I tried cargo install afl and i got errors because clang was missing. Adding such instructions would be great.