[Internal] Refresh PR template (#381)

## What changes are proposed in this pull request?

This PR updates the PR template to remove outdated check boxes and
emphasize testing.

## How is this tested?

N/A

.github/PULL_REQUEST_TEMPLATE.md

@@ -1,6 +1,28 @@
-## Changes
-<!-- Summary of your changes that are easy to understand -->
+## What changes are proposed in this pull request?
 
-## Tests
-<!-- How is this tested? -->
+Provide the readers and reviewers with the information they need to understand
+this PR in a comprehensive manner. 
 
+Specifically, try to answer the two following questions:
+
+- **WHAT** changes are being made in the PR? This should be a summary of the 
+  major changes to allow the reader to quickly understand the PR without having
+  to look at the code. 
+- **WHY** are these changes needed? This should provide the context that the 
+  reader might be missing. For example, were there any decisions behind the 
+  change that are not reflected in the code itself? 
+
+The “why part” is the most important of the two as it usually cannot be 
+inferred from the code itself. A well-written PR description will help future
+developers (including your future self) to know how to interact and update your
+code.
+
+## How is this tested?
+
+Describe any tests you have done; especially if test tests are not part of
+the unit tests (e.g. local tests).
+
+**ALWAYS ANSWER THIS QUESTION:** Answer with "N/A" if tests are not applicable
+to your PR (e.g. if the PR only modifies comments). Do not be afraid of 
+answering "Not tested" if the PR has not been tested. Being clear about what 
+has been done and not done provides important context to the reviewers.