add minimal manpage (help2man ftw)
Install man page to $CMAKE_INSTALL_MANDIR/man1
Write output of `--help` to stdout, as it's expected behaviour

Plus, you can easily pipe it to less that way. Plus, it's a GNU
recommendation:

https://www.gnu.org/prep/standards/html_node/_002d_002dversion.html
--help: Make descriptions lowercase, describe --version/--help
Generate man page at build time using help2man
Merge branch 'feature/manpage'

Conflicts:
	CMakeLists.txt
Add help2man to PKGBUILD dependencies