Add a zsh completion script

Based on @clhodapp's suggestion in https://github.com/spwhitt/nix-zsh-completions/issues/32#issuecomment-705315356 and adapted to use the description of the completions

misc/zsh/completion.zsh

@@ -0,0 +1,21 @@
+function _nix() {
+  local ifs_bk="$IFS"
+  local input=("${(Q)words[@]}")
+  IFS=$'\n'
+  local res=($(NIX_GET_COMPLETIONS=$((CURRENT - 1)) "$input[@]"))
+  IFS="$ifs_bk"
+  local tpe="${${res[1]}%%>	*}"
+  local -a suggestions
+  declare -a suggestions
+  for suggestion in ${res:1}; do
+    # FIXME: This doesn't work properly if the suggestion word contains a `:`
+    # itself
+    suggestions+="${suggestion/	/:}"
+  done
+  if [[ "$tpe" == filenames ]]; then
+    compadd -f
+  fi
+  _describe 'nix' suggestions
+}
+
+compdef _nix nix