Document schema= kwarg in README

Author: effigies

README.md

@@ -1,4 +1,5 @@
 # bids2table
+
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@@ -100,7 +101,6 @@ As an example, here we index all datasets on [OpenNeuro](https://openneuro.org/)
 
 Using 8 threads, we can index all ~1400 OpenNeuro datasets (1.2M files) in less than 15 minutes.
 
-
 ### Indexing datasets from python
 
 You can also index datasets using the Python API.
@@ -129,3 +129,27 @@ pq.write_table(tab, "ds000224.parquet")
 # Convert to a pandas dataframe.
 df = tab.to_pandas(types_mapper=pd.ArrowDtype)
 ```
+
+### Indexing with a custom BIDS schema
+
+By default, `bids2table` uses the BIDS schema bundled with `bidsschematools`.
+Pass a `schema=` argument to `index_dataset`, `batch_index_dataset`,
+`get_arrow_schema`, `get_column_names`, or `validate_bids_entities` to use a
+different schema. The argument may be a path to a schema directory, a string
+URI accepted by `bidsschematools.schema.load_schema`, or a pre-loaded
+`bidsschematools.types.Namespace`.
+
+```python
+import bidsschematools.schema
+import bids2table as b2t2
+
+# Use a pre-loaded schema (e.g. when indexing several datasets that share one).
+schema = bidsschematools.schema.load_schema()
+tab = b2t2.index_dataset("bids-examples/ds102", schema=schema)
+
+# Or pass a path to a custom schema directory.
+tab = b2t2.index_dataset("/data/ds001", schema="/path/to/custom-schema")
+```
+
+Different `schema` arguments may be used for different calls within the same
+process; per-call schemas propagate to worker processes when `max_workers > 0`.