add link between epichains and epidemics about ebola model

[avallecam]

In challenge ebola

You have been tasked to generate initial trajectories of an Ebola outbreak in Guinea. Using model_ebola() and the the information detailed below, complete the following tasks:

We should also add a note to align with the {epichains} episodes, which models Ebola using a branching process (in practice, the two outputs are probably quite similar, but presumably the {epidemics} model doesn't include overdispersion, but can handle herd immunity if large epidemic occurs?)

Originally posted by @adamkucharski in #34 (comment)