Add barebones description of `svelte/register`

I don't know if this is correct, but it works for me and this is better than nothing.

site/content/docs/03-run-time.md

@@ -558,8 +558,17 @@ You can see a full example on the [animations tutorial](tutorial/animate)
 
 ### `svelte/register`
 
-TODO
+To render Svelte components server-side, use `require('svelte/register')`; after this, you can use `require` to include any `.svelte` file.
+
+```js
+require('svelte/register');
+
+const App = require('./App.svelte');
 
+...
+
+App.default.render({ title: 'name' });
+```
 
 ### Client-side component API
 
@@ -569,8 +578,6 @@ TODO
 const component = new Component(options)
 ```
 
----
-
 A client-side component — that is, a component compiled with `generate: 'dom'` (or the `generate` option left unspecified) is a JavaScript class.
 
 ```js