Add README for CI selection validation script

This README explains the validation of CI selections using GlideAjax in a client script, detailing its purpose, usage, and functionality.

Author: hanna-g-sn

Client-Side Components/Catalog Client Script/Validate CI selection via GlideAjax with submit guard/README.md

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+# Validate CI selection via GlideAjax with submit guard
+
+## What this solves
+End users sometimes pick CIs that are retired or unsupported for fulfilment. This client script validates a selected CI using GlideAjax and prevents submission if the CI fails checks.
+This could also be used where some CIs are supported by third parties and prevent use in incorrect forms.
+
+## Where to use
+- Catalog Item with a reference variable to `cmdb_ci`
+- Add the client script as an onChange on that variable
+- Install the Script Include in the same scope
+
+## How it works
+- onChange reads the selected CI sys_id
+- Calls Script Include via GlideAjax
+- Server checks for active CI and support group presence
+- Client displays messages and disables submit if invalid
+
+## References
+- GlideAjax  
+  https://www.servicenow.com/docs/bundle/zurich-api-reference/page/app-store/dev_portal/API_reference/GlideAjax/concept/c_GlideAjaxAPI.html
+- GlideForm client API  
+  https://www.servicenow.com/docs/bundle/zurich-api-reference/page/app-store/dev_portal/API_reference/GlideForm/concept/c_GlideFormAPI.html
+- GlideRecord  
+  https://www.servicenow.com/docs/bundle/zurich-api-reference/page/app-store/dev_portal/API_reference/GlideRecord/concept/c_GlideRecordAPI.html