make fetching URL-PMI optional (closes #33)

index.html

@@ -162,6 +162,30 @@ <h2>
           "!URL#concept-url-equals">equals</a>. [[!URL]]
         </p>
       </section>
+      <section>
+        <h2>
+          Fetching (dereferencing)
+        </h2>
+        <p>
+          As <a>URL-based payment method identifiers</a> serve primarily as
+          identifiers (and not a linked resource), it is OPTIONAL for user
+          agents to <a data-cite="!WHATWG-FETCH#concept-fetch">fetch</a> a
+          <a>URL-based payment method identifier</a>.
+        </p>
+        <p class="note">
+          The working group is in the early stages of standardizing a <a href=
+          "https://w3c.github.io/payment-method-manifest/">Payment
+          Manifest</a>, which will allow developers to describe their payment
+          methods in a machine-processable format. We encourage developers to
+          experiment with Payment Manifest by placing one at the <a>URL-based
+          payment method identifier</a>'s location, but caution that that
+          specification might change in incompatible ways during the
+          standardization process. If you'd like to follow along and
+          contribute, please subscribe to the <a href=
+          "https://github.com/w3c/payment-method-manifest/">payment-method-manifest
+          repository</a> on GitHub.
+        </p>
+      </section>
     </section>
     <section>
       <h2>