doc: add doc for custom datasources (#5548)

doc/rtd/development/datasource_creation.rst

@@ -170,6 +170,8 @@ Datasources included in upstream cloud-init benefit from ongoing maintenance,
 compatibility with the rest of the codebase, and security fixes by the upstream
 development team.
 
+If this is not possible, one can add
+:ref:`custom out-of-tree datasources to cloud-init<custom_datasource>`.
 
 .. _make-mime: https://cloudinit.readthedocs.io/en/latest/explanation/instancedata.html#storage-locations
 .. _DMI: https://www.dmtf.org/sites/default/files/standards/documents/DSP0005.pdf

doc/rtd/reference/base_config_reference.rst

@@ -221,6 +221,8 @@ Other keys
 The :ref:`network configuration<network_config>` to be applied to this
 instance.
 
+.. _base_config_datasource_pkg_list:
+
 ``datasource_pkg_list``
 ^^^^^^^^^^^^^^^^^^^^^^^
 

doc/rtd/reference/custom_modules/custom_datasource.rst

@@ -2,3 +2,18 @@
 
 Custom DataSource
 *****************
+
+Custom 3rd-party out-of-tree DataSources can be added to cloud-init by:
+
+#. :ref:`Implement a DataSource<datasource_creation>` in a Python file.
+
+#. Place that file in as a single Python module or package in folder included
+   in ``$PYTHONPATH``.
+
+#. Extend the :ref:`base-configuration<base_config_reference>`'s
+   :ref:`datasource_pkg_list<base_config_datasource_pkg_list>` to include the
+   Python package where the DataSource is located.
+
+#. Extend the :ref:`base-configuration<base_config_reference>`'s
+   :ref:`datasource_list<base_config_datasource_list>` to include the name of
+   the custom DataSource.