Add a link to the formal specification in TLA+

Author: ligurio

index.html

@@ -301,6 +301,16 @@ <h2>Publications</h2>
   </li>
 </ul>
 
+<a name="spec"></a>
+<h2>Formal specification</h2>
+
+<p>
+This is TLA+ specification for the Raft consensus algorithm, which
+describes Raft in detail: <a href="https://github.com/ongardie/raft.tla">ongardie/raft.tla</a>
+by <a href="https://twitter.com/ongardie">Diego Ongaro</a>. This is slightly
+updated compared to the dissertation version.
+</p>
+
 <a name="talks"></a>
 <h2>Talks</h2>