Add serde

docs/topics/serde.rst

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+Serialization and deserialization
+=================================
+
+The serialization and deserialization system :py:mod:`rosbags.serde` supports multiple raw message formats. For each format it provides a pair of functions, one for serialization and one for deserialization. In addition to the data to process each function usually only requires the message type name.
+
+Deserialization
+---------------
+
+Deserialize a CDR bytes object using :py:func:`deserialize_cdr() <rosbags.serde.deserialize_cdr>`:
+
+.. code-block:: python
+
+   from rosbags.serde import deserialize_cdr
+
+   # rawdata is of type bytes and contains serialized message
+   msg = deserialize_cdr(rawdata, 'geometry_msgs/msg/Quaternion')
+
+Serialization
+---------------
+
+Serialize a message with CDR using :py:func:`serialize_cdr() <rosbags.serde.serialize_cdr>`:
+   
+.. code-block:: python
+
+   from rosbags.serde import serialize_cdr
+
+   # serialize message with system endianess
+   serialized = serialize_cdr(msg, 'geometry_msgs/msg/Quaternion')
+
+   # serialize message with explicit endianess
+   serialized = serialize_cdr(msg, 'geometry_msgs/msg/Quaternion', little_endian=False)