Better introduction to Socsim in the readme

[tomthe]

Bevore the sections on how to install and run rsocsim, we should have a section that explains what socsim and rsocsim actually are.

First, very rough draft:

About Socsim and rsocsim

Socsim stands for Social Simulator. It is a microsimulation software to simulate populations and their kinship structure.

...explanation of what it actually does: Socsim creates a population of individual agents (persons) from input files. It then applies the events "marriage", "childbirth", "death", etc. based on rates that the user provides. It saves the growing kinship network as output.... ...marriage markets? migration? what else?

...explanation of what is possible with this stuff... with examples... users can then analyse the virtual population and the complete kinship structure...

... a bit of history and making clear that this is something established... Socsims first version was developed in the 1970ies in Fortran... rewritten in C in the 80 ... used for countless cool papers throughout the last 50 years... examples...

... and now we (who?) will maintain socsim for the next years. We refactored the code a bit so that it runs on every operating system. We packaged it now into an R-package together with some useful functions that are often needed in working with the in- and output of socsim, so that it is much easier to get started.