Add details on inference methods for fig

[jeromekelleher]

We have no real detail on how the example ARGs fig was generated. Things to address

• Details of the dataset (how many sites, etc, where did we get it?)
• Version numbers of programs?
• What we did to the outputs to turn into tskit gARGs?

This can go in its own paragraph.

Comment from @gregorgorjanc:

We should say how this figure was obtained - I mean, software was run, but every tool produces something different so what was done to represent a comparable gARG figure across methods?

[hyanwong]

I guess I wrote most of the code for this, so it's easiest for me to write this para.

[hyanwong]

Mostly fixed in #398 - remainder to do in #399