MINOR: add missing custom_file_casts example to the list

Signed-off-by: Ruihang Xia <waynestxia@gmail.com>

Author: waynexia

datafusion-examples/README.md

@@ -57,6 +57,7 @@ cargo run --example dataframe
 - [`csv_sql_streaming.rs`](examples/csv_sql_streaming.rs): Build and run a streaming query plan from a SQL statement against a local CSV file
 - [`csv_json_opener.rs`](examples/csv_json_opener.rs): Use low level `FileOpener` APIs to read CSV/JSON into Arrow `RecordBatch`es
 - [`custom_datasource.rs`](examples/custom_datasource.rs): Run queries against a custom datasource (TableProvider)
+- [`custom_file_casts.rs`](examples/custom_file_casts.rs): Implement custom casting rules to adapt file schemas
 - [`custom_file_format.rs`](examples/custom_file_format.rs): Write data to a custom file format
 - [`dataframe-to-s3.rs`](examples/external_dependency/dataframe-to-s3.rs): Run a query using a DataFrame against a parquet file from s3 and writing back to s3
 - [`dataframe.rs`](examples/dataframe.rs): Run a query using a DataFrame API against parquet files, csv files, and in-memory data, including multiple subqueries. Also demonstrates the various methods to write out a DataFrame to a table, parquet file, csv file, and json file.