"Advanced" layout options need documenting

[n8willis]

There's not enough in the README to get started using the Python layout features.

For instance, combing/concatenating layouts.

In the CLI usage, you can definitely do

svg_stack.py --direction=h margin=0 a.svg b.svg c.svg d.svg > row1.svg
svg_stack.py --direction=h margin=0 e.svg f.svg g.svg h.svg > row2.svg

svg_stack.py --direction=v margin=0 row1.svg row2.svg > grid.svg

and get a little

a b c d
e f g h

matrix out.

Trying to replicate that from two ss.HBoxLayouts either doesn't work or else there's some syntax about combining them that isn't clear.

Just trying it straight, making a couple of HBoxLayouts and adding them to a VBoxLayout raises NotImplementedError: rescaling width not implemented (hint: set alignment on file SVGFile('grid.svg'))

...which itself is confusing in two respects: first, I don't ask for any rescaling, so it'd be good to know where that enters into it (and how to configure it), and second, how/where to set an alignment on the output file.

I think this is a fairly straightforward potential use-case, and showing how to do it would help a lot of new users move beyond the 1D example of the CLI.