Update PR template to be more descriptive (#120)

.github/PULL_REQUEST_TEMPLATE.md

@@ -1,6 +1,13 @@
-*Issue #, if available:*
+#### Background
+* What do these changes do?
+* Why are they important?
 
-*Description of changes:*
+#### Testing
+* How did you test these changes?
 
+#### Links
+* Links to additional context, if necessary
+* Issue #, if applicable (see [here](https://docs.github.com/en/issues/tracking-your-work-with-issues/linking-a-pull-request-to-an-issue#linking-a-pull-request-to-an-issue-using-a-keyword) for a list of keywords to use for linking issues)
 
+---
 By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.