Add working pyhdk example -- to readme?

[alexbaden]

e.g.:

import pyhdk
storage = pyhdk.storage.ArrowStorage(1) # 1 is schema id
data_mgr = pyhdk.storage.DataMgr()
data_mgr.registerDataProvider(storage)

calcite = pyhdk.sql.Calcite(storage)
executor = pyhdk.Executor(data_mgr)

import pyarrow
import pandas
at = pyarrow.Table.from_pandas(
            pandas.DataFrame({"a": [1, 2, 3], "b": [10, 20, 30]})
        )
opt = pyhdk.storage.TableOptions(2)
storage.importArrowTable(at, "test", opt)

sql = "SELECT * FROM test;"
ra = calcite.process(sql)
rel_alg_executor = pyhdk.sql.RelAlgExecutor(executor, storage, data_mgr, ra)
print(rel_alg_executor.execute().to_arrow().to_pandas())

print(rel_alg_executor.execute(just_explain = True).to_explain_str())