Resolves 1641 - Checks whether the prompt is set to be managed or not. (#1723)

This is a new variable that will need to be set on all new prompts and
is not backwards compatible with custom prompts that are not prezto
managed, but use prezto's editor-info functionality. Updated the
README.md with additional information for themes.

modules/prompt/functions/prompt-pwd

@@ -28,3 +28,4 @@ unset current_pwd
 print "$ret_directory"
 
 # }
+# vim: ft=zsh