Add PR template

Author: rictic

.github/PULL_REQUEST_TEMPLATE

@@ -0,0 +1,16 @@
+<!--
+  Thanks for the PR!
+
+  If this change has a user visible change (including
+  bug fixes, new features, etc) please describe the change in
+  CHANGELOG.md.
+
+  If the change is an entirely package-internal reshuffling/refactoring
+  should the change not be described in the CHANGELOG.
+
+  Consider also updating the README.
+
+  More info: http://keepachangelog.com/en/0.3.0/
+ -->
+
+ - [ ] CHANGELOG.md has been updated