add a little more information about computation to readme

[jscamac]

@wolass tried to reproduce paper at a recent reproducibility hacka-thon in Berlin. Appears analysis and figures were reproduced but the pdf of the manuscript would not compile (see his issue).

He pointed out that the analysis was slow (too ages). This is largely because its running 11 stan (bayesian models). I can make this clearer within the Readme. Computation varies depending on system and specs. But tests generally are ~ 1 hour.

[wolass]

On my laptop using ubuntu i7 4200 U 8 GB RAM the computation alone took 1:25 h, rendering of the paper <10 secs.