Add man page

[dvzrv]

Hi! I'm packaging desync for Arch Linux.

I noticed that this project does not offer a man page for the desync executable.
This would be very helpful to have, so that people do not have to call the actual executable to read about its options.
Additionally, we (and other distributions, too) have a man page indexing service, which allows to read the man pages of packages online: https://man.archlinux.org
The man page for desync would automatically end up there and can serve as a reference point for further documentation (e.g. in our wiki or in other places).

[folbricht]

Great, that would be a good addition to have for sure. I haven't written man pages before but initially we could use the content of the various help texts. Is there a certain layout/structure that would help you package them? I wouldn't mind if you made a PR just for the outline of what it should look like and I fill in the content.

[dvzrv]

Sorry, I'm swamped with so many other things...

Generally, I guess it would be great if the man page is either generated (e.g. from markdown or something similar) or available statically (far less readable) and can be installed to the default locations using a Makefile.

A Makefile should honor $DESTDIR (default to nothing) and allow configuring $PREFIX (default to /usr/local).
The man page can then be installed to $(DESTDIR)/$(PREFIX)/share/man/man1/.

[folbricht]

#232 adds a new 'manpage' command which lets you generate them. Does that work for your build-system? Or would you prefer I generate a set and commit into the code?