opam: Add documentation and remove opam-installer

Signed-off-by: Euan Harris <euan.harris@citrix.com>
Merge pull request #507 from euanh/opam

opam: Add documentation and remove opam-installer
opam: Put back opam-installer!

Signed-off-by: Euan Harris <euan.harris@citrix.com>
Merge pull request #508 from euanh/opam

opam: Put back opam-installer!
opam: Fix typo in changelog

Signed-off-by: Euan Harris <euan.harris@citrix.com>