Update pull_request_template.md

Author: neetcode-gh

.github/pull_request_template.md

@@ -9,3 +9,7 @@
 [//]: # "Go to the leetcode [`Submissions tab`](https://user-images.githubusercontent.com/71089234/180188604-b1ecaf90-bf27-4fd6-a559-5567aebf8930.png)"
 [//]: # "and [click on the `Accepted` status of your submission.](https://user-images.githubusercontent.com/71089234/180189321-1a48c33f-aa65-4b29-8aaa-685f4f5f8c9e.png)]"
 [//]: # "Finally copy the URL from the nav bar, it should look like https://leetcode.com/problems/[problem-name]/submissions/xxxxxxxxx/"
+
+
+### Important
+Please make sure the file name is lowercase and a duplicate file does not already exist before merging.