Document SSR support and behavior

[PeppeL-G]

It took some time to search for, but after a while I found #56 (comment):

By working server side, I mean it should not cause an error.

Local Storage is a client-side browser API, so on the server it returns the default value of the store. Please advise if you are seeing a different behavior.

Would it be possible to mention this in the docs (in the README file)? I guess most use Svelte with SvelteKit, and in SvelteKit (with SSR) this is a quite important piece of information.