add section on contactmatrix package to contact-matrices

[amanda-minter]

          I would maybe leave the different methods for generating contact matrices for later and then describe in more detail (and probably at some stage introduce the contactmatrix package, once we have a common contact matrix class, which is work in progress).

Contact matrices can be estimated from diary data. For example, the POLYMOD survey measured contact patterns in 8 European countries using data on the location and duration of contacts reported by the study participants [(Mossong et al. 2008)](https://doi.org/10.1371/journal.pmed.0050074).

Originally posted by @sbfnk in #52 (comment)