install opam on ubunti code example

Example: install opam on centos

yum install sqlite-devel fuse fuse-devel libcurl-devel zlib-devel m4
    yum install ocaml ocamldoc ocaml-camlp4-devel
    wget [https://raw.github.com/ocaml/opam/master/shell/opam_installer.sh](https://raw.github.com/ocaml/opam/master/shell/opam_installer.sh) -O - | sh -s /usr/local/bin/
    yum install