Merge pull request #281 from w3c/onix08

Added note with text from comment in 271

Author: GeorgeKerscher

UX-Guide-Metadata/draft/principles/index.html

@@ -196,6 +196,9 @@ <h3>Metadata techniques</h3>
 						<p><a href="../techniques/onix-metadata/index.html">ONIX Accessibility Metadata</a></p>
 					</li>
 				</ul>
+<div class="note">
+<p>Publishers update their ONIX records as needed. We expect "unknown" accessibility metadata may be initially provided but may change as more information becomes available. For this reason, implementors should be prepared to update the accessibility metadata as new ONIX feeds are made available.</p>
+</div>
 			</section>
 
 		</section>