Move test generation code into README (deephaven#4965)

This (hopefully) fixes a problem where GH security is alerting that we are using an insecure version of pyarrow (<14.0.1).

extensions/parquet/table/README.md

@@ -0,0 +1,66 @@
+# extensions-parquet-table
+
+## Test data
+
+Some of the test data under [src/test/resources](src/test/resources) was generated with the following snippet:
+
+```python
+import pandas as pd
+import numpy as np
+
+df = pd.DataFrame(
+    {
+        "a": list("abc"),
+        "b": list(range(1, 4)),
+        "c": np.arange(3, 6).astype("u1"),
+        "d": np.arange(4.0, 7.0, dtype="float64"),
+        "e": [True, False, True],
+        "f": pd.date_range("20130101", periods=3),
+        "g": pd.date_range("20130101", periods=3, tz="US/Eastern"),
+        "h": pd.Categorical(list("abc")),
+        "i": pd.Categorical(list("abc"), ordered=True),
+    }
+)
+
+df.to_parquet("uncompressed.parquet", compression=None)
+df.to_parquet("brotli.parquet", compression="brotli")
+df.to_parquet("gzip.parquet", compression="gzip")
+df.to_parquet("lz4.parquet", compression="lz4")
+df.to_parquet("snappy.parquet", compression="snappy")
+df.to_parquet("zstd.parquet", compression="zstd")
+```
+
+Using the following requirements:
+
+```requirements
+# for src/test/resources/e0
+numpy==1.24.2
+pandas==1.5.3
+pyarrow==5.0.0
+python-dateutil==2.8.2
+pytz==2022.7.1
+six==1.16.0
+```
+
+```requirements
+# for src/test/resources/e1
+numpy==1.24.2
+pandas==1.5.3
+pyarrow==11.0.0
+python-dateutil==2.8.2
+pytz==2022.7.1
+six==1.16.0
+```
+
+```requirements
+# for src/test/resources/e2
+cramjam==2.6.2
+fastparquet==2023.2.0
+fsspec==2023.3.0
+numpy==1.24.2
+packaging==23.0
+pandas==1.5.3
+python-dateutil==2.8.2
+pytz==2022.7.1
+six==1.16.0
+```