DOC: Table.save() store hash for parquet files (#446)

* DOC: Table.save() store hash for parquet files

* Provide code example

* Update example

* Mention that audb uses the hash

* Discuss reasons why md5 sum differs

audformat/core/table.py

@@ -586,6 +586,28 @@ def save(
 
         Existing files will be overwritten.
 
+        When using ``"parquet"`` as ``storage_format``
+        a hash,
+        based on the content of the table,
+        is stored under the key ``b"hash"``
+        in the metadata of the schema of the parquet file.
+        This provides a deterministic hash for the file,
+        as md5 sums of parquet files,
+        containing identical information,
+        often differ.
+        Reasons include factors like the library
+        that wrote the parquet file,
+        the chosen compression codec
+        and metadata written by the library.
+
+        The hash can be accessed with ``pyarrow`` by::
+
+            pyarrow.parquet.read_schema(f"{path}.parquet").metadata[b"hash"].decode()
+
+        The hash is used by :mod:`audb`
+        when publishing a database
+        to track changes of database files.
+
         Args:
             path: file path without extension
             storage_format: storage format of table.