suggesting to improve reading example in readme

[vaifrax]

I started from the example in the readme. Unfortunately, error handling is not part of the example, this would be useful. But I consider it essential to include the clean up part, e.g. like this:

// done, clean up
heif_image_handle_release(handle);
heif_context_free(ctx);

At least this would have saved me a lot of debugging.

Thanks for your effort!

[vaifrax]

plus

heif_image_release(img);

[quiian]

Yes, please. Glad I found this :)