Add example for specifying theme file

See issue #61 for my confusion that lead to adding this example here.
Actually, if you are referring to a built-in theme, you can just specify it by name:

```
$ ./bin/asciidoctor-pdf -a pdf-style=asciidoctor examples/example.adoc
```

You only need the full path if you are referring to a theme at another location. Even then, you can use the `pdf-stylesdir` attribute to specify the location of themes. If the `pdf-style` value does not end in `.yml`, then the file name is calculated as `{pdf-style}-theme.yml` located in the `pdf-stylesdir` directory (which defaults to the built-in `data/themes` directory).