patch 8.2.3608: users who type "q:" instead of ":q" are confused

Problem:    Users who type "q:" instead of ":q" are confused.
Solution:   Add an autocmd to give a message that explains this is the
            command-line window. (Egor Zvorykin, closes #9146)

runtime/defaults.vim

@@ -111,6 +111,17 @@ if 1
 
   augroup END
 
+  " Quite a few people accidentally type "q:" instead of ":q" and get confused
+  " by the command line window.  Give a hint about how to get out.
+  " If you don't like this you can put this in your vimrc:
+  " ":augroup vimHints | au! | augroup END"
+  augroup vimHints
+    autocmd! CmdwinEnter *
+	  \ echohl Todo | 
+	  \ echo 'You discovered the command-line window! You can close it with ":q".' |
+	  \ echohl None
+  augroup END
+
 endif
 
 " Switch syntax highlighting on when the terminal has colors or when using the

src/testdir/test_autocmd.vim

@@ -2458,6 +2458,7 @@ func Test_autocmd_CmdWinEnter()
   CheckFeature cmdwin
 
   let lines =<< trim END
+    augroup vimHints | au! | augroup END
     let b:dummy_var = 'This is a dummy'
     autocmd CmdWinEnter * quit
     let winnr = winnr('$')

src/testdir/test_cmdline.vim

@@ -1255,6 +1255,7 @@ func Test_cmdwin_restore()
   CheckScreendump
 
   let lines =<< trim [SCRIPT]
+    augroup vimHints | au! | augroup END
     call setline(1, range(30))
     2split
   [SCRIPT]

src/version.c

@@ -757,6 +757,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    3608,
 /**/
     3607,
 /**/