Add note about static VC JSON-LD Context and vocab. See #206.

Author: msporny

index.html

@@ -2165,7 +2165,12 @@ <h2>JSON-LD</h2>
 processing a <a>verifiable credential</a> or <a>verifiable presentation</a>,
 and a <code>@context</code> property is not present at the top-level of the
 JSON-LD document, then a <code>@context</code> property with a value of
-<code>https://w3.org/2018/credentials/v1</code> MUST be assumed.
+<code>https://w3.org/2018/credentials/v1</code> MUST be assumed. The
+JSON-LD Context available at <code>https://w3.org/2018/credentials/v1</code>
+is a static document that is never updated and may thus be downloaded and
+cached client side. The associated vocabulary document for the
+Verifiable Credentials Data Model can be found at the following URL:
+<code>https://w3.org/2018/credentials</code>.
         </p>
 
       </section>