Documentation: move sharedsubtrees.txt to filesystems/

This documentation is also vfs-related.
Signed-off-by: NJ. Bruce Fields <bfields@citi.umich.edu>
Acked-by: NRandy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

Documentation/00-INDEX

@@ -358,8 +358,6 @@ sgi-visws.txt
 	- short blurb on the SGI Visual Workstations.
 sh/
 	- directory with info on porting Linux to a new architecture.
-sharedsubtree.txt
-	- a description of shared subtrees for namespaces.
 smart-config.txt
 	- description of the Smart Config makefile feature.
 sony-laptop.txt

Documentation/filesystems/00-INDEX

@@ -82,6 +82,8 @@ relay.txt
 	- info on relay, for efficient streaming from kernel to user space.
 romfs.txt
 	- description of the ROMFS filesystem.
+sharedsubtree.txt
+	- a description of shared subtrees for namespaces.
 smbfs.txt
 	- info on using filesystems with the SMB protocol (Win 3.11 and NT).
 spufs.txt