Update pull_request_template.md

Draft updates to distinguish implementor commitments for shipped vs. unshipped features.

Author: sanketj

.github/pull_request_template.md

@@ -1,13 +1,14 @@
 <!-- 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:
- * [ ] At least two implementers are interested:
+ * [ ] Editing WG resolution on the proposed changes
+
+ * [ ] Implementation interest for features that have yet to be shipped in all browsers (link to issue, or write "Implementing" or "Not Implementing"):
    * [ ] WebKit
    * [ ] Chromium
    * [ ] Gecko
- * [ ] Editing WG resolution on the proposed changes
 
-Implementation bugs are filed:
- * [ ] 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=)
+ * [ ] Implementation bugs for browsers that are shipping the feature:
+   * [ ] 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=)