Add paragraph on using the DID Spec Registries for DID parameters.

See this comment #36 (comment).

index.html

@@ -914,6 +914,14 @@ <h2>DID Parameters</h2>
           </tbody>
         </table>
 
+        <p>
+Implementers as well as <a>DID method</a> specification authors MAY use additional
+DID parameters that are not listed here.
+For maximum interoperability, it is RECOMMENDED that DID parameters use the official
+W3C DID Specification Registries mechanism [[DID-SPEC-REGISTRIES]], to avoid collision
+with other uses of the same DID parameter with different semantics.
+        </p>
+
         <p>
 Additional considerations for processing these parameters are discussed in
 [[?DID-RESOLUTION]].