Add deprecation notice on @position-fallback

Author: jamesnw

index.html

@@ -387,6 +387,11 @@ <h2>
         </div>
       </div>
       <div class="note">
+        <p>
+          <strong>Note: </strong>The <code>@position-fallback</code> syntax has
+          been replaced in the spec, and will be deprecated in the next version
+          of this Polyfill.
+        </p>
         <p>
           With polyfill applied, the following positions are attempted in order:
         </p>