[Fix #753] Add a note for helm-M-x

README.md

@@ -416,6 +416,10 @@ If you prefer Ido in everywhere, you should not add `prelude-helm-everywhere`, s
 
 You can always reactivate Helm with `(prelude-global-helm-global-mode-on)`.
 
+**NOTICE**: In `helm-M-x`, you have to pass prefix argument *AFTER* you run `helm-M-x`,
+because your prefix argument will be displayed in the modeline when in `helm-M-x`
+buffer. Passing prefix argument **BEFORE** =helm-M-x= **has no effect**.
+
 
 #### Key-chords