Document key conflict with viper

doc/org.texi

@@ -10929,6 +10929,18 @@ fixed this problem:
 This package also uses the @kbd{S-<cursor>} keys, so everything written
 in the paragraph above about CUA mode also applies here.
 
+@item @file{viper.el} by Michael Kifer
+@cindex @file{viper.el}
+@kindex C-c /
+Viper uses @kbd{C-c /} and therefore makes this key not access the
+corresponding Org-mode command @code{org-sparse-tree}.  You need to find
+another key for this command, or override the key in
+@code{viper-vi-global-user-map} with
+
+@lisp
+(define-key viper-vi-global-user-map "C-c /" 'org-sparse-tree)
+@end lisp
+
 @end table