Add Prelude tip on keyboard shortcut to join lines

core/prelude-core.el

@@ -392,6 +392,7 @@ Doesn't mess with special buffers."
     "Press <C-c C-z> in a Elisp buffer to launch an interactive Elisp shell."
     "Press <C-Backspace> to kill a line backwards."
     "Press <C-S-Backspace> or <s-k> to kill the whole line."
+    "Press <s-j> or <C-^> to join lines."
     "Press <f11> to toggle fullscreen mode."
     "Press <f12> to toggle the menu bar."
     "Explore the Tools->Prelude menu to find out about some of Prelude extensions to Emacs."