Add Linus' optimized sha1 hash for checksum

It does appear to be pretty fast.

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
diff --git a/HOWTO b/HOWTO
index aec00e8..bc23052 100644
--- a/HOWTO
+++ b/HOWTO
@@ -794,6 +794,8 @@
 
 			sha256	Use sha256 as the checksum function.
 
+			sha1	Use optimized sha1 as the checksum function.
+
 			meta	Write extra information about each io
 				(timestamp, block number etc.). The block
 				number is verified.