add parquet example (#80)

Change-Id: Id642fcfdf4cd2e8a8bc7a1f01d596082bad0ca59

Author: wangfenjin

examples/int32_decimal.parquet

@@ -0,0 +1,16 @@
+extern crate duckdb;
+use duckdb::{Connection, Result};
+
+use arrow::record_batch::RecordBatch;
+use arrow::util::pretty::print_batches;
+
+fn main() -> Result<()> {
+    let db = Connection::open_in_memory()?;
+    db.execute_batch("INSTALL parquet; LOAD parquet;")?;
+    let rbs: Vec<RecordBatch> = db
+        .prepare("SELECT * FROM read_parquet('./examples/int32_decimal.parquet');")?
+        .query_arrow([])?
+        .collect();
+    assert!(print_batches(&rbs).is_ok());
+    Ok(())
+}