Add PR template

Author: mbyczkowski

.github/PULL_REQUEST_TEMPLATE.md

@@ -0,0 +1,13 @@
+## Pull Request Checklist
+
+**Is this in reference to an existing issue?**
+
+#### General
+
+- [ ] Update Changelog following the conventions laid out on [Keep A Changelog](http://keepachangelog.com/)
+
+- [ ] Update README with any necessary configuration snippets
+
+- [ ] Existing tests pass
+
+#### Purpose