Add specific calculation of Rt from data

[fccoelho]

@maxbiostat you need to add a section illustrating how to go from the data to the actual Beta prime distribution of the Rt. I think this should go in the main manuscript.

[fccoelho]

We can mention in the main text that the criteria for the elicitation of the prior for the binomial parameter, $a_0$ and $b_0$, are show in the supplementary information.