Add README for Incident Priority Set on Insert Only

Added documentation for the Incident Priority Set on Insert Only script, explaining its purpose, usage, and functionality.

Author: hanna-g-sn

Server-Side Components/Transform Map Scripts/Incident Priority Set on Insert Only/README.md

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+# Incident priority set on insert only
+
+## What this solves
+Recurring imports often overwrite priority even after the service desk has triaged the ticket. This onBefore script sets priority only when a row is inserted. Updates pass through without touching priority.
+
+## Where to use
+Attach to your Incident Transform Map as an onBefore script.
+
+## How it works
+- Checks the Transform Map action variable for insert vs update
+- On insert, computes priority from impact and urgency
+- On update, does nothing to priority
+
+## References
+- Transform Map scripts  
+  https://www.servicenow.com/docs/bundle/zurich-integrate-applications/page/administer/import-sets/task/t_AddOnBeforeScriptToTransformMap.html
+- Incident fields and priority logic  
+  https://www.servicenow.com/docs/bundle/zurich-it-service-management/page/product/incident-management/concept/incident-fields.html