Added an issue box to the install prompts section.

The issue is that the spec doesn't currently let the user agent fire
BeforeInstallPrompt without being forced to show an install prompt if the event
is not cancelled.

index.html

@@ -451,6 +451,16 @@ <h2>
             </ol>
           </li>
         </ol>
+        <div class="issue">
+          Implementations may wish to show a prompt if and only if the site
+          explicitly requests it via
+          <a><code>BeforeInstallPromptEvent.prompt()</code></a>, but not
+          automatically without the site's approval. Is this something we want
+          to leave to the discretion of the user agent? (This would require
+          changing the language here, as "steps to notify before an automated
+          install prompt" would become "steps to notify that an install prompt
+          is available".)
+        </div>
       </section>
       <section>
         <h3 id="installation-sec">