Add note about schema.org terms being considered stable by the WG.

index.html

@@ -6030,6 +6030,17 @@ <h3>Vocabularies</h3>
 `curl -sL -H "Accept: &lt;MEDIA_TYPE>" &lt;DOCUMENT_URL> | openssl dgst -&ltDIGEST_ALGORITHM> -binary | openssl base64 -nopad -a`
         </p>
 
+        <p class="note"
+            title="schema.org changes regularly, but considered stable">
+Implementers and document authors might note that cryptographic digests for
+`schema.org` are not provided. This is due to the `schema.org` vocabulary
+undergoing regular changes; any digest provided would be out of date within
+weeks of publication. The Working Group discussed this concern and concluded
+that the vocabulary terms that are used from `schema.org`, by this
+specification, have been stable for years and are highly unlikely to change in
+their semantic meaning.
+        </p>
+
         <p>
 The following base classes are defined in this specification for processors
 and other specifications that benefit from such definitions: