Update build instructions

Small pull request to update the build instructions to include `opam install ocamlfind`. Since I was doing this from never having installed any ocaml packages it turned out that `opam install llvm` didn't finish the trick. I was receiving the error:

```shell
ocamlyacc parser.mly
ocamlfind ocamlc -g -w @5@8@10@11@12@14@23@24@26@29@40 -c -package llvm -linkpkg ast.mli
make: ocamlfind: No such file or directory
make: *** [ast.cmi] Error 1
```

Running `opam install ocamlfind` certainly fixed that.