documenting supported operating systems

[Christian Brabandt]

Hi,
I found it hard to know, what kind of operating systems are supported or
when support for older versions has been dropped. So I started
documenting the current state in vi_diff.txt at :h os-support (requires
this Commit:
https://github.com/vim/vim/commit/18defabaebe3b8e679b5df2d7c8ec82172a6e41b)

Please have a look and if you can think of any improvements, let me know
(or send a patch ;) )

Thanks,
Christian
--
This is a country where people are free to practice their religion,
regardless of race, creed, color, obesity, or number of dangling keys...