Add pull request template (#1093)

* Add pull request template

* Add bit about testing

.github/pull_request_template.md

@@ -0,0 +1,20 @@
+### Description
+
+_What's included in this PR?_
+
+### Drive-by changes
+
+_Are there any minor or drive-by changes also included?_
+
+### Related issues
+
+- Fixes #[issue number here]
+
+### Backward compatibility
+
+_Are these changes backward compatible?_
+_Are there any infrastructure implications, e.g. changes that would prohibit deploying older commits using this infra tooling?_
+
+### Testing
+
+_What kind of testing has these changes undergone?_