Document how to use import maps

[Rhal95]

Document how to bundle with import maps.

When I follow the readme and write a simple script to bundle my application I get something like this:

import { bundle } from "https://deno.land/x/emit@0.35.0/mod.ts";

const url: string = import.meta.resolve('./main.ts');
const { code } = await bundle(url);

// write 'code' into a file 

Which works as long as we are not using import maps in the application.

If you try to use import maps you will just get a cryptic error:

error: Uncaught (in promise) Error: Unable to output during bundling.
      const ret = new Error(getStringFromWasm0(arg0, arg1));

Passing the used import map into the bundle() function will solve this:
const { code } = await bundle(url, {importMap: './deno.json'});

This should be documented in the readme. I just spend an hour trying to figure out why my build suddenly stopped working after updating a couple of dependencies and introducing import maps.