Add a notice inside admin dashboard so users know they need to regenerate thumbnails for theme v1.3.x

[cristianraiber]

Introduced in Activello 1.3 to fix: #81 & #74

Should recommend this plugin: https://wordpress.org/plugins/force-regenerate-thumbnails/

[cristeacosmin]

After installing - activating the plugin, i still have a button with no action attached to it. It should link to the plugin options or start regenerating thumbs