Update pull_request_template.md

.github/pull_request_template.md

@@ -2,5 +2,7 @@
 
 1. PLEASE CAREFULLY READ: [Contributing Guidelines](https://github.com/ocornut/imgui/blob/master/docs/CONTRIBUTING.md)
 
-2. Clear this template before submitting your PR.
+2. Make sure you're using a special branch just for this pull request. (Sometimes people unknowingly use a default branch, then later update that branch, which updates the pull request with the other changes if it hasn't been merged yet.)
+
+3. Clear this template before submitting your PR.