Remove ‘post-thumbnails’ prerequisite from readme

The [Codex page](http://codex.wordpress.org/Function_Reference/add_image_size) for the `add_image_size` function is incorrect. To add additional image sizes there is no need for the theme to explicitly support post-thumbnails.
Merge pull request #6 from barryceelen/patch-2

Remove ‘post-thumbnails’ prerequisite from readme