Add pull-request template

Author: Fokko

.github/pull_request_template.md

@@ -0,0 +1,14 @@
+<!--
+Thanks for opening a pull request!
+-->
+
+<!-- In the case this PR will resolve an issue, please replace ${GITHUB_ISSUE_ID} below with the actual Github issue id. -->
+<!-- Closes #${GITHUB_ISSUE_ID} -->
+
+# Rationale for this change
+
+# Are these changes tested?
+
+# Are there any user-facing changes?
+
+<!-- In the case of user-facing changes, please add the changelog label. -->