efivarfs: Add documentation for the EFI variable filesystem

Signed-off-by: Matt Fleming <matt.fleming@intel.com>

Documentation/filesystems/00-INDEX

@@ -38,6 +38,8 @@ dnotify_test.c
 	- example program for dnotify
 ecryptfs.txt
 	- docs on eCryptfs: stacked cryptographic filesystem for Linux.
+efivarfs.txt
+	- info for the efivarfs filesystem.
 exofs.txt
 	- info, usage, mount options, design about EXOFS.
 ext2.txt

Documentation/filesystems/efivarfs.txt

+
+efivarfs - a (U)EFI variable filesystem
+
+The efivarfs filesystem was created to address the shortcomings of
+using entries in sysfs to maintain EFI variables. The old sysfs EFI
+variables code only supported variables of up to 1024 bytes. This
+limitation existed in version 0.99 of the EFI specification, but was
+removed before any full releases. Since variables can now be larger
+than a single page, sysfs isn't the best interface for this.
+
+Variables can be created, deleted and modified with the efivarfs
+filesystem.
+
+efivarfs is typically mounted like this,
+
+	mount -t efivarfs none /sys/firmware/efi/efivars