Add plugin for LXD autocomplete (#7457)

plugins/lxd/README.md

@@ -0,0 +1,9 @@
+# lxd
+
+This plugin provides completion for [lxd](https://linuxcontainers.org/lxd/), as well as aliases
+for frequent lxc commands.
+
+To use it add `lxd` to the plugins array in your zshrc file.
+
+```zsh
+plugins=(... lxd)