Add demonstration of using Encoding to serialize strings for stdio/stdout/stderr

I think it'd be helpful to include a second example in the readme that demonstrates encoding strings to UTF8 to pass to stdin, and how to decode stdout/stderr.

Here's a basic example I figured out after a bit of trial and error: https://gist.github.com/ioben/4b7ec3d0b8cbfadbfbcf6e905102ef4f

I think it'd be important to at least demonstrate this, it's a very common use-case. It would be awesome to provide facilities to do this conversion automatically too.