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/
Add OpenBSD to list of OSes in INSTALL.txt

This should be updated again when OpenBSD 5.7 comes out, around June 2015.
Git doesn't support "orange"; use "yellow".

6e6508408b7ee7e595d657bba0fc30de076fae4c set the default colour for modified
files to:

    git config --get-color "gitsh.color.modified" "orange"

Unfortunately, there is no "orange". The colour has always been called
"yellow", it just looks orange in preferred terminal emulator colour scheme.

Oops.