Add some clarifications to the README on build dependencies

- Minimum OCaml compiler version is 4.01.0+, not 4.x
- OPAM package may also be convenient for some users

You may not want all the detail in the latter, but I thought it
would be useful for those users stuck on ancient distros like
CentOS that ship with OCaml 3.12.1.

The related OPAM pull request: ocaml/opam-repository#3083