List all supported platforms in the readme

[janosh]

Would be great if you could see at a glance from the readme if sveltekit-embed has a component for a given platform.

[spences10]

Hey @janosh yeah, I think I had something like that but it turns out it's just a Tweet 😅

https://twitter.com/spences10/status/1521444473212788897

Feel free to make a PR, I'll probably need this on the example site too

[spences10]

Added the platforms to the readme with 7028778

[janosh]

Thanks!