Add signals (v1) to list of discontinued libraries

libs/libraries.htm

@@ -90,6 +90,9 @@ how to download, build, and install the libraries.</p>
         from Nicolai Josuttis.  Removed in Boost version 1.32.
         Please use <a href="bind/bind.html">Bind</a> or <a
         href="lambda/index.html">Lambda</a> instead.</li>
+    <li>signals (v1) - Managed signals and slots callback implementation, from
+        Doug Gregor. Removed in Boost version 1.69. Please use
+        <a href="signals2/index.html">Signals2</a> instead.</li>
 </ul>
 
 <hr>