followup: develop COVID-19 specific examples in vignette and readme

[mjrolland]

The documentation of the follow-up package (ffinger/followup) so far consists of function documentation and a technical description alongside with a few generic examples in a vignette.
It would be a great addition to develop a few COVID-19 specific examples to demonstrate a potential use-case in a separate vignette (and the readme) and in the readme using actual realistic parameters.

[impact: Contact tracing is an essential part of the COVID-19 response in many countries and of infectious diseases in general. Regular (phone) followup of known contacts to ask them if they have developed symptoms improves can improve the performance of the procedure since contacts who develop symptoms can immediately be directed towards triage and testing facilities, isolation and medical care, decreasing the risk for them to infect others. With high numbers of reported cases the number of contacts in many countries becomes difficult to manage. This package provides a way to objectively prioritize contacts to be followed-up according to the probability that they have yet developed symptoms.]
[originally proposed by @ffinger]
[suggested repo: https://github.com/ffinger/followup | @ffinger]
[additional notes: ]