command-not-found: add README (#7272)

plugins/command-not-found/README.md

+# command-not-found plugin
+
+This plugin uses the command-not-found package for zsh to provide suggested packages to be installed if a command cannot be found.
+
+To use it, add `command-not-found` to the plugins array of your zshrc file:
+
+```zsh
+plugins=(... command-not-found)
+```
+
+An example of how this plugin works in Ubuntu:
+```
+$ mutt
+The program 'mutt' can be found in the following packages:
+ * mutt
+ * mutt-kz
+ * mutt-patched
+Try: sudo apt install <selected package>
+```
+
+### Supported platforms
+
+It works out of the box with the command-not-found packages for:
+
+- [Ubuntu](https://www.porcheron.info/command-not-found-for-zsh/)
+- [Debian](https://packages.debian.org/search?keywords=command-not-found)
+- [Arch Linux](https://wiki.archlinux.org/index.php/Pkgfile#Command_not_found)
+- [macOS (Homebrew)](https://github.com/Homebrew/homebrew-command-not-found)
+- [Fedora](https://fedoraproject.org/wiki/Features/PackageKitCommandNotFound)
+
+You can add support for other platforms by submitting a Pull Request.