docs/library/marshal: Document the marshal module.

Signed-off-by: Damien George <damien@micropython.org>

docs/library/index.rst

@@ -69,6 +69,7 @@ library.
    heapq.rst
    io.rst
    json.rst
+   marshal.rst
    math.rst
    os.rst
    platform.rst

docs/library/marshal.rst

@@ -0,0 +1,28 @@
+:mod:`marshal` -- Python object serialization
+=============================================
+
+.. module:: marshal
+   :synopsis: Convert Python objects to and from a binary format
+
+|see_cpython_module| :mod:`python:marshal`.
+
+This module implements conversion between Python objects and a binary format.
+The format is specific to MicroPython but does not depend on the machine
+architecture, so the data can be transferred and used on a different MicroPython
+instance, as long as the version of the binary data matches (it's currently
+versioned as the mpy file version, see :ref:`mpy_files`).
+
+Functions
+---------
+
+.. function:: dumps(value, /)
+
+   Convert the given *value* to binary format and return a corresponding ``bytes``
+   object.
+
+   Currently, code objects are the only supported values that can be converted.
+
+.. function:: loads(data, /)
+
+   Convert the given bytes-like *data* to its corresponding Python object, and
+   return it.