Add missing upgrade note to the 2.1

UPGRADE-2.1.md

@@ -1,5 +1,55 @@
 # UPGRADE FROM `2.1.8` TO `2.1.9`
 
+## Telemetry
+
+Sylius 2.1.9 introduces anonymous telemetry to help us understand how Sylius is used and improve the platform.
+
+**What data is collected:**
+- Anonymous installation ID (hashed, non-reversible)
+- Sylius and PHP versions, default locale
+- Aggregated statistics as segments (broad ranges, not exact values):
+   - Customers/products/variants count (e.g., "1K-10K", "100K-1M")
+   - GMV and AOV ranges per month (e.g., "100K-500K", "50-100")
+
+**No sensitive data is ever collected** - no customer information, no order details, no personal data.
+
+**Configuration:**
+
+Telemetry is enabled by default and uses a default salt for hashing the installation ID.
+
+To disable telemetry, set the following environment variable in your `.env` file:
+
+```dotenv
+SYLIUS_TELEMETRY_ENABLED=0
+```
+
+To change the salt, set the `SYLIUS_TELEMETRY_SALT` environment variable:
+
+```dotenv
+SYLIUS_TELEMETRY_SALT=your-custom-salt
+```
+
+**Database migration (optional):**
+
+This release includes an optional database migration that adds an index to improve telemetry query performance.
+The telemetry system works without this index, but adding it will make data collection faster, especially for stores with large order volumes.
+
+To run the migration:
+
+```bash
+php bin/console doctrine:migrations:migrate
+```
+
+If you want to skip this migration:
+
+```bash
+php bin/console doctrine:migrations:migrate --exclude-migrations="Sylius\\Bundle\\CoreBundle\\Migrations\\Version20251126120000"
+# For PostgreSQL:
+php bin/console doctrine:migrations:migrate --exclude-migrations="Sylius\\Bundle\\CoreBundle\\Migrations\\Version20251126120001"
+```
+
+For more details, see the [Telemetry documentation](https://docs.sylius.com/the-book/configuration/telemetry).
+
 ## Autocomplete Form Types
 
 1. The `choice_value => 'code'` option has been removed from autocomplete form types (`ProductAutocompleteType`,