clarify that the default Pull Request description should not be deleted (#2788)

.github/PULL_REQUEST_TEMPLATE.md

@@ -1,3 +1,5 @@
+<!-- DO NOT DELETE THE TEXT BELOW. Please make sure relevant boxes are checked [x] -->
+
 Thank you for taking the time to work on a PR for Awesome-Selfhosted!
 
 To ensure your PR is dealt with swiftly please check the following: