Recommend to have why in PR desc (#540)

# Summary

Adding why a particular PR is needed, is important in PR desc. This help
reviewers understand the PR better.
This PR encourages authors to do that by updating the PR template.

## Proof of Work
NA

Author: viveksinghggits

.github/pull_request_template.md

@@ -1,6 +1,6 @@
 # Summary
 
-<!-- Enter your issue summary here.-->
+<!-- Enter your PR summary here. Try to emphasize on WHY this change is needed, followed by what's being done in the PR. -->
 
 ## Proof of Work