* etc/PROBLEMS: Add entry about dir-locals and some auto-mounters.

etc/PROBLEMS

 Known Problems with GNU Emacs
 
-Copyright (C) 1987-1989, 1993-1999, 2001-2015 Free Software Foundation,
-Inc.
+Copyright (C) 1987-1989, 1993-1999, 2001-2015 Free Software Foundation, Inc.
 See the end of the file for license conditions.
 
 
@@ -469,6 +468,18 @@ full qualified domain name, FQDN.  You should have your FQDN in the
 
 The way to set this up may vary on non-GNU systems.
 
+*** Visiting files in some auto-mounted directories causes Emacs to print
+`Error reading dir-locals: (file-error "Read error" "is a directory" ...'
+
+This can happen if the auto-mounter mistakenly reports that
+.dir-locals.el exists and is a directory.  There is nothing Emacs can
+do about this, but you can avoid the issue by adding a suitable entry
+to the variable `locate-dominating-stop-dir-regexp'.  For example, if
+the problem relates to "/smb/.dir-locals.el", set that variable
+to a new value where you replace "net\\|afs" with "net\\|afs\\|smb".
+(The default value already matches common auto-mount prefixes.)
+See http://lists.gnu.org/archive/html/help-gnu-emacs/2015-02/msg00461.html .
+
 *** Attempting to visit remote files via ange-ftp fails.
 
 If the error message is "ange-ftp-file-modtime: Specified time is not