Add attach instructions for duckdb iceberg integration

DuckDB-Iceberg-Integration.md

@@ -118,6 +118,25 @@ SELECT * FROM iceberg_scan('s3://my-catalog/my-namespace/my-table');
 SELECT * FROM iceberg_scan('s3://my-catalog/my-namespace/my-table/metadata/v1.metadata.json');
 ```
 
+## "Attaching" the catalog
+
+Once secrets are configured, it is possible to attach the catalog for direct access:
+
+```sql
+ATTACH 's3://my-catalog' AS my_catalog(
+  TYPE iceberg,
+  SECRET iceberg_secret,
+  ENDPOINT 'http://localhost:8181',
+  READ_ONLY false
+);
+
+SELECT * FROM my_catalog.my-namespace.my-table;
+
+INSERT INTO my_catalog.my-namespace.my-table VALUES (...);
+```
+
+Note that remote catalogs are mounted read-only by default, so if you need to be able to modify things, you need the `READ_ONLY false`. If you only need to read, no need to add the `READ_ONLY false`.
+
 ## Configuration Reference
 
 | Parameter | Description | Default |