Add 'ready reckoners' vignette

A useful tool during COVID was the 'ready reckoner' plot by [Ellen Brooks-Pollock et al](https://pmc.ncbi.nlm.nih.gov/articles/PMC8165600/), which mapped social distancing scenarios to the reproduction number - see below image as example from this paper. By combining `socialmixr` data by setting with `r_eff()` from {finalsize}, this should be relatively straightforward to include as an example for users for basic social distancing (but some more thought would be required to incorporate assumptions about contact tracing etc, perhaps linking with {ringbp} development).