docs/autoconf: Mark autoconf as being replaced

I know it's not what anyone wants, but how about we start with a
message in the documentation that encourages people to try meson.

Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>
Reviewed-by: Eric Engeström <eric@engestrom.ch>

docs/autoconf.html

@@ -26,6 +26,12 @@
   </ul>
 </ol>
 
+<h2>ATTENTION:</h2>
+<p>
+    The autotools build is being replaced by the <a href="meson.html">meson</a>
+    build system. If you haven't yet now is a good time to try using meson and
+    report any issues you run into.
+</p>
 
 <h2 id="basic">1. Basic Usage</h2>