kube-ps1: add example for common pitfall (#8318)

I ran into this issue, looks like maybe a few other people did as well.

https://github.com/jonmosco/kube-ps1/issues/28#issuecomment-359761463
A simple clarification seems helpful.

plugins/kube-ps1/README.md

@@ -55,6 +55,8 @@ plugins=(
 
 # After the "source Oh My Zsh" line
 PROMPT=$PROMPT'$(kube_ps1) '
+# Or in double quotes, don't forget to escape
+PROMPT=$PROMPT"\$(kube_ps1) "
 ```
 
 Note: The `PROMPT` example above was tested with the theme `robbyrussell`.