Add known issues section.

base_multi_image/README.rst

@@ -59,6 +59,13 @@ To develop a module based on this one:
    :alt: Try me on Runbot
    :target: https://runbot.odoo-community.org/runbot/149/8.0
 
+Known issues / Roadmap
+======================
+
+* *OS file* storage mode for images is meant to provide a path where Odoo has
+  read access and the image is already found, **not for making the module store
+  images there**. It would be nice to add that feature though.
+
 Bug Tracker
 ===========