Concat Example

Hi Alan,

thanks for your markdown tools.
However, it took me a while to figure out that in case of `concat:true` then `dest` should be a single file, not a directory. On later investigation, I found [your example](https://github.com/alanshaw/markdown-pdf#concat-from-multiple-paths-to-single-path), but it would be easier if you could provide a similar example in the README.md of this project.

```
grunt.initConfig({
markdownpdf: {
options: { concat:true },
files: {
src: "src/*.md",
dest: "dest/output.pdf"
}
}
})
```

Best regards,
Björn