Update README: Add section on Server Side Rendering

[limelights]

Added a quick example on how to enable SSR in svelte-loader since documentation was missing this.
It should ideally say what all values of generate do and how they work but ssr is the first value I stumbled upon.

[0gust1]

All the options you pass to the loader, that are not in this list below, will be passed to the Svelte compiler.

const pluginOptions = {
	externalDependencies: true,
	hotReload: true,
	hotOptions: true,
	preprocess: true,
	emitCss: true,

	// legacy
	shared: true,
	style: true,
	script: true,
	markup: true
};

To make that more explicit, we could make a paragraph in the README explaining that, and link to the compiler readme (for the compiler options).

Imho, explaining compiler's options in the loader's readme is not very sustainable and mixes concerns.

[AlbertMarashi]

On another note regarding SSR documentation, I'm willing to create an extensive SSR guide (including support for code-splitting). To do code-splitting with SSR you need a client & server plugin, which I've built and happy to add to the svelte-loader core for anyone who wants first-class support for server-side rendering and preloading/prefetch of asynchronous scripts (for faster loading speeds)

More information: #124

[nergmada]

Is the ability to include the options: { generate: 'ssr' } still available?

I read the note about mixing concerns and I agree but I think it might be worth a reference like "any options here will be passed to svelte.compile, see svelte.compile for more" then there's a sense of direction. Sorry don't need some rando's two cents, but there you go.

[benmccann]

We've moved this under compilerOptions in #159

[benmccann]

We should make these docs generically talk about the Svelte compiler options rather than just SSR specifically since there might be other options people would want to use as well - though we could use SSR in the example

[benmccann]

Added to the readme in #161