Document base component

[benmccann]

None of the framework docs such as https://unpic.pics/img/svelte/ show the transformer prop being used, which I believe is now required. I'm not sure if there are other changes that might be required as well

[ascorbic]

No, transformer is only required when using the new base component. The normal component has the same required props as before.

[benmccann]

Oh, I see. The base component doesn't seem to be documented. It would be nice to mention that you can also import from @unpic/svelte/base