Documentation: move rpc-cache.txt to filesystems/

This file is nfs-related.  (Maybe Documentation/filesystems/ would
benefit from a separate nfs/ directory at some point.)

Signed-off-by: J. Bruce Fields <bfields@citi.umich.edu>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
diff --git a/Documentation/filesystems/00-INDEX b/Documentation/filesystems/00-INDEX
index 2ec174c..52cd611 100644
--- a/Documentation/filesystems/00-INDEX
+++ b/Documentation/filesystems/00-INDEX
@@ -84,6 +84,8 @@
 	- info on relay, for efficient streaming from kernel to user space.
 romfs.txt
 	- description of the ROMFS filesystem.
+rpc-cache.txt
+	- introduction to the caching mechanisms in the sunrpc layer.
 seq_file.txt
 	- how to use the seq_file API
 sharedsubtree.txt