Add test case suggestion to PR template

Summary: Pull Request resolved: #21234

Differential Revision: D9982500

Pulled By: hramos

fbshipit-source-id: 585d4a8292ac7f53f26adf53a36e766a49fdc5b6

Author: hramos

.github/PULL_REQUEST_TEMPLATE.md

@@ -1,8 +1,10 @@
-Thank you for sending the PR! We appreciate you spending the time to work on these changes. 
+Thank you for sending the PR! We appreciate you spending the time to work on these changes.
 Help us understand your motivation by explaining why you decided to make this change.
 
 If this PR fixes an issue, type "Fixes #issueNumber" to automatically close the issue when the PR is merged.
 
+_Pull requests that expand test coverage are more likely to get reviewed. Add a test case whenever possible!_
+
 Test Plan:
 ----------
 Write your test plan here. If you changed any code, please provide us with clear instructions on how you verified your changes work. Bonus points for screenshots and videos!