following README for getting HTML string didn't work for me

I only got markdown when I was trying to get HTML. By putting `console.log` statements in the code, I noticed that the default config had:
```
{ marked: undefined, html: undefined }
```

I ended up just doing an asset transformation, which is what originally looking for. Would be nice if how to do that were more clearly explained in the README. Here's the example I came up with that seems to work for me:
https://github.com/ultrasaurus/hello-parcel-markdown/tree/simple-asset-transform

basically, just needed to add `.markedrc` with
```
{
"marked": true,
"html": true
}

then `yarn parcel src/index.md`

Start with the README instructions for getting an HTML string and compare them with the working asset-transformation example linked in the issue. Verify the documented `.markedrc` settings and `yarn parcel src/index.md` command; done means a newcomer can follow the README to obtain HTML instead of only markdown.

Written by the indexing model from the issue text.