ci: add PR template (#249)

Author: Alex Plischke

.github/PULL_REQUEST_TEMPLATE.md

@@ -0,0 +1,6 @@
+## Description
+<!--
+Describe the big picture of your changes here to communicate to the maintainers
+why we should accept this pull request. If it fixes a bug or resolves a feature
+request, be sure to link to that issue.
+-->