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
Merge pull request #12 from avsm/README-tweaks

Add some clarifications to the README on build dependencies
 Mention flow init when doesn't find .flowconfig

fixes #19.

I'm not sure what the policy is on multiline error messages, but this looked a little better on separate lines IMO.  I tested the change locally.

I'm also not sure if this extra message is needed once people become familiar with the tool (or look at the --help options).  I found out pretty quickly I needed to run `flow init`, so I'm guessing others will as well.  Feel free to ignore/close this pull request.  I figured it was a ticket I could close out quickly (when looking at the issues).
Merge pull request #27 from skratchdot/patch-1

 Mention flow init when doesn't find .flowconfig