docs: add upgrade notice

README.md

@@ -20,3 +20,10 @@ spec:
         parameters:
           shared_preload_libraries: "vectors.so"
 ```
+
+> :warning: If you want to do a major Posgres version upgrade like described in the [official doc](https://access.crunchydata.com/documentation/postgres-operator/latest/guides/major-postgres-version-upgrade), make sure you do the following steps or the upgrade will fail:
+
+1. Connect to the database where you have enabled the `vectors` extension and disable it with `DROP EXTENSION vectors;`.
+2. Remove the `dynamicConfiguration` block you added to `PostgresCluster` from above.
+3. Follow the upgrade instruction from the official doc.
+4. Add back the removed block and enable the extension in the database again with `CREATE EXTENSION vectors;`