Add hash seed to verify_slice()

Author: garious

README.md

@@ -24,7 +24,7 @@ Create a *Historian* and send it *events* to generate an *event log*, where each
 is tagged with the historian's latest *hash*. Then ensure the order of events was not tampered
 with by verifying each entry's hash can be generated from the hash in the previous entry:
 
-![historian](https://user-images.githubusercontent.com/55449/36492299-151c65e2-16ea-11e8-97b6-14f1d9b97985.png)
+![historian](https://user-images.githubusercontent.com/55449/36492930-97a572be-16eb-11e8-8289-358e9507189e.png)
 
 ```rust
 extern crate silk;

diagrams/historian.msc

@@ -18,5 +18,5 @@ msc {
   logger=>historian [ label = "e2 = Entry{hash: h6, n: 3, event: Tick}" ] ;
   client=>historian [ label = "collect()" ] ;
   historian=>client [ label = "entries = [e0, e1, e2]" ] ;
-  client=>client [ label = "verify_slice(entries)" ] ;
+  client=>client [ label = "verify_slice(entries, h0)" ] ;
 }