Updte a README file

fs/mount/fs_mount.c

@@ -280,7 +280,7 @@ int mount(FAR const char *source, FAR const char *target,
 
       if (INODE_IS_SPECIAL(mountpt_inode))
         {
-         fdbg("ERROR: target %s exists and is a special nodes\n", target);
+          fdbg("ERROR: target %s exists and is a special nodes\n", target);
           errcode = -ENOTDIR;
           goto errout_with_semaphore;
         }

fs/unionfs/README.txt

@@ -52,6 +52,20 @@ fs/unionfs/README.txt
   with a writable file system (like a RAM disk).  This should then give
   to a readable/write-able file system with some fixed content.
 
+  Prefixes
+  --------
+
+  And optional prefix may be provided with each of the file systems
+  combined in by the Union File System.  For example, suppose that:
+
+  o File system 1 is a ROMFS file system with prefix == NULL,
+  o File system 2 is a BINFS file system with prefix == "cgin-bin", and
+  o The union file system is mounted at /mnt/www.
+
+  Then the content in the in the ROMFS file system would appear at
+  /mnt/www and the content of the BINFS file system would appear at
+  /mnt/www/cgi-gin.
+
   Example Configurations
   ----------------------