Mention the contextvars dependency in the CHANGELOG

Fixes #23.

CHANGELOG.rst

@@ -16,7 +16,11 @@ Change log
 
 - Changed the ``verbose_name`` of fields generated by
   ``TranslatedField`` to return the language_code when inside a
-  ``with translated_fields.show_language_code(True):`` block.
+  ``with translated_fields.show_language_code(True):`` block. This
+  change introduces a dependency on `contextvars
+  <https://docs.python.org/3/library/contextvars.html>`__ which is
+  automatically installed from `PyPI
+  <https://pypi.org/project/contextvars/>`__ in Python<3.7.
 - Completely overhauled ``TranslatedFieldAdmin`` to take advantage of
   ``show_language_code``, making it possible to use translated fields
   together with ``list_display_links``, ``list_editable``,