Add README for MRVS normalisation client script

This README explains how to normalise and reset MRVS rows based on changes to a controlling variable, detailing setup and usage.

Author: hanna-g-sn

Client-Side Components/Catalog Client Script/Normalise and Reset a MRVS based on Variable Changes/README.md

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+# MRVS - Normalise and Reset Rows on Change
+
+## What this solves
+When a controlling variable changes (for example, Environment), existing MRVS rows may no longer be valid. This client script:
+- Clears or normalises specific MRVS columns
+- Deduplicates rows
+- Optionally sorts rows for a cleaner UX
+- Works entirely client-side using MRVS JSON
+
+## Where to use
+Catalog Item → OnChange client script on your controlling variable.
+
+## How it works
+- Reads the MRVS value as JSON via `g_form.getValue('my_mrvs')`
+- Applies transforms (clear columns, unique by key, sort)
+- Writes back the JSON with `g_form.setValue('my_mrvs', JSON.stringify(rows))`
+
+## Setup
+1. Replace `CONTROLLING_VARIABLE` with your variable name.
+2. Replace `MY_MRVS` with your MRVS variable name.
+3. Adjust `COLUMNS_TO_CLEAR`, `UNIQUE_KEY`, and `SORT_BY` as needed.
+
+## Notes
+- To clear the MRVS entirely, set `rows = []` before `setValue`.
+- Works with Catalog Client Scripts; no server call required.
+
+## References
+- GlideForm API (client): `getValue`, `setValue`, `clearValue`  
+  https://www.servicenow.com/docs/bundle/zurich-api-reference/page/app-store/dev_portal/API_reference/GlideForm/concept/c_GlideFormAPI.html
+- Working with MRVS values on the client (community examples)  
+  https://www.servicenow.com/community/developer-articles/accessing-multi-row-variable-set-value-outside-the-multi-row/ta-p/2308876