📝 Add note about ticking checkboxes directly

Author: thibmaek

.github/PULL_REQUEST_TEMPLATE.md

@@ -1,4 +1,9 @@
 # Please make sure all below checkboxes have been checked before submitting your PR.
+<!-- IMPORTANT:
+  If you can check some boxes, please submit your PR first and then
+  tick the boxes in the PR description. Don't place x'es in the square brackets [] directly.
+  Thank you ✨
+-->
 
 - [ ] I have read and understood the [contribution guidelines](https://github.com/thibmaek/awesome-raspberrypi/blob/master/CONTRIBUTING.md).
 - [ ] This pull request has a descriptive title. *(For example: `Add Raspbian`)*