Add documentation about writing records

Author: twiggler

docs/source/tools/rdump.rst

@@ -120,7 +120,27 @@ For example, we can output just the hostname, name and image path of a Windows s
 Writing records
 ---------------
 
-Something about writing records, e.g. auto detection of filename for compression.
+``rdump`` can write records to a file, which can be used as input for ``rdump`` at a later stage. To write
+records to a file, the ``-w`` or ``--writer`` argument can be used.
+
+The output format and compression type are automatically detected based on the filename extension. For example, to
+write to a gzip compressed file, simply use the ``.gz`` extension in your output file.
+
+.. code-block:: console
+
+    $ rdump services.rec -w services.rec.gz
+
+This will read the records from ``services.rec`` and write them to a new gzip compressed file named ``services.rec.gz``.
+Other supported compression formats are ``.bz2``, ``.zst`` (zstandard), and ``.lz4``.
+
+If you want to write the records to a file without any compression, just use a filename without a compression
+extension.
+
+.. code-block:: console
+
+    $ rdump services.rec -w services.rec.out
+
+If no ``-w`` argument is provided, ``rdump`` writes the records to standard output in its default line-based format.
 
 Usage
 -----