Add docs about (minimal) public API

Created by: z0u

I use scour as a module in a web app. Previously, I was doing this:

    opts = scour.parse_args(args=[...])[0]
    output = scour.scourString(svg, opts)

But that fails in version 0.35, because parse_args now returns a single object instead of a tuple. The fix is easy; however, it would be nice to have a public API that would remain stable for a given major version number.