chore: update pull request template (#141)

Author: trivikr

.github/PULL_REQUEST_TEMPLATE.md

@@ -1,5 +1,19 @@
-_Issue #, if available:_
+### Issue
 
-_Description of changes:_
+Issue number, if available, prefixed with "#"
+
+### Description
+
+What does this implement/fix? Explain your changes.
+
+### Testing
+
+How was this change tested?
+
+### Additional context
+
+Add any other context about the PR here.
+
+---
 
 By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of your choice.