Document RxJS support

[Rich-Harris]

Need to add to the docs the fact that you can now use RxJS/TC39 observables like regular stores

[MVSICA-FICTA]

Great, this means I get to dive into yet another web component related framework in the coming week! Just finished lit-html and lit-element and had a pretty good feeling there already.

I think my main question will revolve around injection options. Can Svelte components be used as functions and be dynamically injected at runtime? I had discovered the open-wc lit-element demos and there were some good strategies for component composition via factories and the like there as well. It would be good to document these different composition and injection options too.

Hardly had time to look at Svelte but at least it gets rid of the most obvious 'render' function declaration and now supports RxJS.

[TehShrike]

#2549 (comment)

[evdama]

Regarding docs, we have one REPL that has a code snippet https://svelte.dev/repl/51144b5ef95b53d14aede232d58aa14b?version=3.1.0

There's another quite similar snippet to be found here
https://bl.ocks.org/rveciana/0fdb54cb77770c3695425b8f633c7b3e

It might also make sense to elaborate a bit on how this relates to the overall store contract such as discussed here for example #4216

@kylecordes @trbrc do you think the REPL snippet from above is a suitable demo and could be used for documenting the use of RxJS with svelte? I'm new to Svelte and RxJS so I really don't feel competent enough to have an oppinioun how a few lines of text and a code snippet should look like in order to count as documentation...

Last but not least, in order to clarify things, when I can use the $observable$ syntax with svelte markup such as

{#await $observable$}
  pending - No value or error has been received yet
{:then result}
  fulfilled - Received a value
{:catch error}
  rejected - Received an error
{/await}

isn't something like https://github.com/timhall/svelte-observable now actually redundant to that native $observable$ syntax approach that we now have in Svelte?