Add small note to PR template

[ci skip]

.github/pull_request_template.md

@@ -2,7 +2,7 @@
 
 <!-- Type some lines about your submission -->
 
-
+<!-- If you are not submitting a new scheme, we suggest removing the following lines -->
 #### Manually checked properties
 <!-- These checkboxes serve for the maintainers of PQClean to verify your submission. Please do not check them yourself. -->