Add release note block to PR template (knative#586)

This is inspired by the Kubernetes PR template's release note block. The
intent is that release managers (or a bot) can collate these notes into
the changelog on release. The `release-note` syntax annotation is not a
real syntax, but is intended to make this easier to automate by scanning
PR bodies.

.github/pull-request-template.md

@@ -2,6 +2,13 @@ Fixes #
 
 ## Proposed Changes
 
-  *  
-  * 
-  * 
+  *
+  *
+  *
+
+**Release Note**
+<!-- Enter your extended release note in the below block. If the PR requires
+additional action from users switching to the new release, include the string
+"action required". If no release note is required, write "NONE". -->
+```release-note
+```