input: signal end of file, goon

Hi I had the following line of code:

```
Porcelain.exec("wkhtmltopdf", ["-", "-"], [in: "some html", out: :string])
```

but after this line the pgm blocked waiting forever for an answer. I tried with wc -l and it is the same pbm, I suggested it was because porcelain coulnd'nt send EOF signal. I tested to install goon, and it worked.
It took me quite some time to understand that goon was necessary for this feature.
I was expecting it to work out of the box. Maybe you should explicitely tells that you can't use custom input output without goon when displaying "goon executable not found" and/or enumerate the features that can"t work without goon?