f2fs: add document

This adds a document describing the mount options, proc entries, usage, and
design of Flash-Friendly File System, namely F2FS.
Signed-off-by: Jaegeuk Kim <jaegeuk.kim@samsung.com>

Documentation/filesystems/00-INDEX

@@ -48,6 +48,8 @@ ext4.txt
 	- info, mount options and specifications for the Ext4 filesystem.
 files.txt
 	- info on file management in the Linux kernel.
+f2fs.txt
+	- info and mount options for the F2FS filesystem.
 fuse.txt
 	- info on the Filesystem in User SpacE including mount options.
 gfs2.txt