added an example of collecting file records

doc/source/ch08_csst_common.rst

@@ -139,6 +139,27 @@ Use ``FileRecorder.add_record()`` to add file records.
     test001.txt True Test file 1     False
     test002.txt True Test file 2     False
 
+`FileRecorder` inherits from `list`.
+Therefore, if you are collecting records from subprocesses,
+here is the way to combine records:
+
+.. code-block:: python
+    :linenos:
+
+    >>> from csst_common.file_recorder import FileRecorder
+    >>> fr1 = FileRecorder()
+    >>> fr1.add_record(filepath="test1.txt", db=False, comment="test file 1")
+    >>> fr2 = FileRecorder()
+    >>> fr2.add_record(filepath="test2.txt", db=False, comment="test file 2")
+    >>> fr_all = FileRecorder([*fr1, *fr2])
+    >>> fr_all.pprint_all()
+
+    <FileRecorder length=2>
+     filepath   db    comment   existence
+    --------- ----- ----------- ---------
+    test1.txt False test file 1     False
+    test2.txt False test file 2     False
+
 
 ``csst_common.logger.get_logger()``
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