Add a man page

[fingolfin]

A simple manpage would be useful to have. These are pretty easy to make these days when using e.g. asciidoc (the tool git.git uses).

As a bonus, if we install a man page for git-remote-gitifyg, it will be displayed by git help remote-gitifyhg.

We could also install a manpage under the name "gitifyhg" for extra convenience.