OpenSSH: add notes for which versions of OpenSSH ship with various OSes by default

[fingolfin]

Use a "note" (see issue #9) to enhance the OpenSSH implementation page to list which versions of OpenSSH ship with which OS versions. So that one can quickly find out that Mac OS X ships OpenSSH 5.9p1 (perhaps even give the full output of openssh -v), or that "Ubuntu 14.04.2 LTS" ships OpenSSH 6.6.1p1, etc.

The idea being that this way, one can quickly check which versions of OpenSSH (and hence which features, weaknesses, etc.) are available where. This can be helpful when one wants to e.g. establish company wide SSH policies (enabling / disabling certain protocols).

[fingolfin]

The release notes at http://www.openssh.com/txt/ are helpful for figuring this out.