doc(readme): mention NetBSD pkg
(ref: https://pkgsrc.se/x11/xst)
| Arch Linux (AUR) | =xst-git= |
| Fedora (COPR) | =keefle/xst= |
| Gentoo (personal overlay) | https://framagit.org/3/ebuilds |
+| NetBSD | =xst= |
| Void Linux | =xst= |
*** Thanks
