Update ractive.render()

Updates render() with basic usage info.

The docs say that `render()` will throw an error, but it doesn't. In fact, calling `insert()` on a new component will ask you to invoke `render()`.

```
var r = new Ractive({ template: '<div>hi</div>' });
r.insert(document.body); // fail

var parent = document.createDocumentFragment(); /* or .createElement('DIV') */
r.render(parent);
r.insert(document.body); // ok
```