Updated README to document used CSS custom properties. (#11)

It's required to set these properties in order for the editor controls
to correctly render.

README.rst

@@ -151,3 +151,17 @@ If you're rendering the form in a template you have to include the form media:
 
 Note that the form media isn't django-prose-editor specific, that's a Django
 feature.
+
+The django-prose-editor CSS uses the following CSS custom properties, using
+names that are defined in the Django admin's CSS:
+
+* ``--border-color``
+* ``--body-fg``
+* ``--body-bg``
+* ``--primary``
+
+You should set these properties with appropriate values to use
+django-prose-editor outside the admin in your site.
+
+In addition, you may optionally set a ``--prose-editor-typographic`` property
+to control the color of typographic characters when shown.