docs: Add OpenBSD port

Author: phy1729

INSTALL.md

@@ -9,6 +9,7 @@ How to install
 * FreeBSD: `pkg install zsh-syntax-highlighting` (port name: [`shells/zsh-syntax-highlighting`][freebsd-port])
 * Gentoo: [mv overlay][gentoo-overlay]
 * Mac OS X / Homebrew: [brew install zsh-syntax-highlighting][brew-package]
+* OpenBSD: `pkg_add zsh-syntax-highlighting` (port name: [`shells/zsh-syntax-highlighting`][openbsd-port])
 * openSUSE / SLE: `zsh-syntax-highlighting` package in [OBS repository][obs-repository]
 * RHEL / CentOS / Scientific Linux: `zsh-syntax-highlighting` package in [OBS repository][obs-repository]
 * Ubuntu: `zsh-syntax-highlighting` package [in Xenial][ubuntu-package] (or in [OBS repository][obs-repository])
@@ -23,6 +24,7 @@ How to install
 [freebsd-port]: http://www.freshports.org/textproc/zsh-syntax-highlighting/
 [gentoo-overlay]: http://gpo.zugaina.org/app-shells/zsh-syntax-highlighting
 [obs-repository]: https://software.opensuse.org/download.html?project=shells%3Azsh-users%3Azsh-syntax-highlighting&package=zsh-syntax-highlighting
+[openbsd-package]: https://cvsweb.openbsd.org/ports/shells/zsh-syntax-highlighting/
 [ubuntu-package]: https://launchpad.net/ubuntu/+source/zsh-syntax-highlighting
 [void-package]: https://github.com/void-linux/void-packages/tree/master/srcpkgs/zsh-syntax-highlighting