update pull_request_templete

.github/pull_request_template.md

@@ -0,0 +1,10 @@
+<!--
+If this PR is related to an unstable feature or an otherwise tracked effort,
+please link to the relevant tracking issue here. If you don't know of a related
+tracking issue or there are none, feel free to ignore this.
+
+This PR will get automatically assigned to a reviewer. In case you would like
+a specific user to review your work, you can assign it to them by using
+
+    r​? <reviewer name>
+-->