spec: Added a non-normative section discussing extending the API.

Author: mgiuca

index.html

@@ -388,6 +388,60 @@ <h2>
         </li>
       </ul>
     </section>
+    <section class="appendix informative">
+      <h2>
+        Extensibility of this API
+      </h2>
+      <p>
+        The Web Share API is designed to be extended in the future by way of
+        new members added to to the <a>ShareData</a> dictionary, to allow both
+        sharing of new types of data (<i>e.g.</i>, <a href=
+        "https://github.com/WICG/web-share/issues/12">images</a>) and strings
+        with new semantics (<i>e.g.</i> author).
+      </p>
+      <div class="warning">
+        This doesn't mean user agents can add whatever members they like. It
+        means that new members can be added to the standard in the future.
+      </div>
+      <p>
+        The three members <a for="ShareData">title</a>, <a for=
+        "ShareData">text</a>, and <a for="ShareData">url</a>, are part of the
+        base feature set, and implementations that provide
+        <a><code>navigator.share</code></a> need to accept all three. Any new
+        members that are added in the future will be <em>individually
+        feature-detectable</em>, to allow for backwards-compatibility with
+        older implementations that don't recognise those members. These new
+        members might also be added as optional "MAY" requirements.
+      </p>
+      <div class="note">
+        There is <a href="https://github.com/heycam/webidl/issues/107">an open
+        discussion</a> about how to provide feature-detection for dictionary
+        members. Web Share will use the mechanism produced by that discussion.
+      </div>
+      <p>
+        The <a for="Navigator"><code>share</code></a> method throws a
+        <a data-cite=
+        "!WEBIDL#exceptiondef-typeerror"><code>TypeError</code></a> if none of
+        the specified members are present. The intention is that when a new
+        member is added, it will also be added to this list of recognised
+        members (so as to not throw an error). This is for future-proofing
+        implementations: if a web site written against a future version of this
+        spec uses <em>only</em> new members (<i>e.g.</i>,
+        <code>navigator.share({image: x})</code>), it will be valid in future
+        user agents, but a <a data-cite=
+        "!WEBIDL#exceptiondef-typeerror"><code>TypeError</code></a> on user
+        agents implementing an older version of the spec. Developers will be
+        asked to feature-detect any new fields they rely on, to avoid having
+        errors surface in their program.
+      </p>
+      <p>
+        Editors of this spec will want to carefully consider the genericity of
+        any new members being added, avoiding fields that are closely
+        associated with a particular service, user agent or operating system,
+        in favour of fields that can potentially be applied to a wide range of
+        platforms and targets.
+      </p>
+    </section>
     <section class="appendix">
       <h2>
         Acknowledgments