Make BeforeInstallPrompt optional (#797)

Author: Marcos Cáceres

index.html

@@ -603,10 +603,14 @@ <h3>
         </p>
       </section>
     </section>
-    <section>
+    <section class="atrisk">
       <h2>
         Installation Events
       </h2>
+      <p>
+        Installation events and supporting the {{BeforeInstallPrompt}} is
+        OPTIONAL.
+      </p>
       <p>
         DOM events <a>fired</a> by this specification use the <dfn>application
         life-cycle task source</dfn>.