Add help message for async VCS to prompt

.config/zsh/functions/prompt_tastytea_setup

@@ -1,8 +1,8 @@
 # tastytea prompt theme
-# Asynchronous VCS info if zsh-async is detected.
 
 prompt_tastytea_help () {
     cat <<'EOF'
+Minimal + vcs_info on the left, PWD on the right (shortened to 1/3 screen).
 This prompt is colour-scheme-able. You can invoke it thus:
 
   prompt tastytea [user [root [remote [vcs [path [highlight [inactive]]]]]]]
@@ -10,6 +10,10 @@ This prompt is colour-scheme-able. You can invoke it thus:
 Set DEFAULT_USER to only show non-default users in prompt. You need to set up
 vcs_info before enabling the prompt.
 EOF
+  if [[ ! -v ASYNC_VERSION ]]; then
+      print "Load https://github.com/mafredri/zsh-async" \
+            "for asynchronous VCS info support."
+  fi
 }
 
 # Reduce directory elements to first letter until directory path is reasonably