Document required Netlify CMS configuration

[axelboc]

If I'm not mistaken, the plugin assumes that the configured Netlify CMS public_folder is one level down from the media_folder - e.g.

media_folder: "static/uploads"
public_folder: "/uploads"

I'm referring more specifically to line 129:

imagePath = path.join(file.dir, '..', value);

If that's the case, it might be worth mentioning it in the README.

[danielmahon]

@axelboc I think you're right, probably better to use path.basename rather than jumping up a level.

[axelboc]

Ah yeah, great solution!