make plugin: Ammended documentation to note it's since version.

This is a :mod:`BuildElement <buildstream.buildelement>` implementation for

using GNU make based build.

.. note::

   The ``make`` element is available since :ref:`format version 9 <project_format_version>`

Here is the default configuration for the ``make`` element in full:

  .. literalinclude:: ../../../buildstream/plugins/elements/make.yaml