Update README.md

```js
var roaster = require("roaster");
var fs = require("fs");
var options = {}

roaster("./markdown.md", options, function(err, contents) {
    fs.writeFileSync("./markdown.html", contents, "utf8");
});
```

The above snippet is supposed to read from a file then write from a file, however based on the following quote from the README this should try and convert the string `./markdown.md` to markdown before writing it to a HTML document.

> If you pass options.isFile, the first parameter is assumed to be a file path. By default, this is false, which means that the first parameter is an actual string of Markdown.

The proposed changes fixes this and also adds a snippet for converting Markdown to HTML.