Document CR exit criteria and duration

as discussed in w3c/transitions#491

index.bs

@@ -28,6 +28,13 @@ Status Text: <p class="note" role="note">
   further improvements are expected to be reflected in revised Candidate
   Recommendation Drafts and Snaphots.
   </p>
+  <p>Before requesting transition to <a href="https://www.w3.org/standards/types#PR">Proposed Recommendation</a>, the Working Group will seek to demonstrate that:</p>
+  <ul>
+  <li>the API is  implementable on top of existing major platform APIs, such as Android Neural Networks API, Windows DirectML, and macOS/iOS Metal Performance Shaders and Basic Neural Network Subroutines;</li>
+  <li>it has at least two independent, interoperable implementations of every feature defined in the specification, where interoperability can be verified by passing open test suites, and two or more implementations interoperating with each other;</li>
+  <li>it has an open test suite of every feature defined in the specification.</li>
+  </ul>
+  <p>The Working Group does not expect to request transition until 1 October 2023.</p>
 </pre>
 <pre class="anchors">
 urlPrefix: https://gpuweb.github.io/gpuweb/; spec: WEBGPU