Update PR template per Editing WG resolution.

.github/PULL_REQUEST_TEMPLATE.md

@@ -1,12 +1,13 @@
-Closes #????
+<!-- Please fill in the sections below when making normative changes. Feel free to remove the sections when only making non-normative changes. -->
 
 For normative changes, the following tasks have been completed:
-
- * [ ] Modified Web platform tests (link to pull request)
-
-Implementation commitment:
-
- * [ ] WebKit (https://bugs.webkit.org/show_bug.cgi?id=)
- * [ ] Chromium (https://bugs.chromium.org/p/chromium/issues/detail?id=)
- * [ ] Gecko (https://bugzilla.mozilla.org/show_bug.cgi?id=)
+ * [ ] Editing WG resolution on the proposed changes, with at least two implementers participating and not objecting:
+   * [ ] WebKit
+   * [ ] Chromium
+   * [ ] Gecko
+
+ * [ ] For browsers that are shipping the feature, implementation bugs are filed for the proposed changes (link to bug, or write "Not Implementing"):
+   * [ ] WebKit (https://bugs.webkit.org/show_bug.cgi?id=)
+   * [ ] Chromium (https://bugs.chromium.org/p/chromium/issues/detail?id=)
+   * [ ] Gecko (https://bugzilla.mozilla.org/show_bug.cgi?id=)