project: add title instructions to PR template

.gitea/pull_request_template.md

@@ -1,3 +1,11 @@
+<!--
+Please include the main mod this PR affects in the title, including
+the leading directory. For example, if you have added a new
+type of banner to mcl_banners, the title should look like:
+
+    items/mcl_banners: add new banner type
+
+-->
 ## Problem
 TRACKING ISSUE: #<!-- Tracking issue number -->