Add OpenBSD instructions

Removed the `On ` prefix in the instructions to reduce redundancy.

The gitsh port and package are in OpenBSD 5.7+, which is not yet
released, so the instructions target -current. The package mirrors are
not even caught up yet but they hopefully will be soon.

http://cvsweb.openbsd.org/cgi-bin/cvsweb/ports/devel/gitsh/