create README

[lwarode]

• README file should contain basic information of the package and deliver a low level introduction
• some elements might be a duplicate in vignette or vice versa, but rather focus on basic elements of package and don't show too complex workflow mechanisms
• implement table that is showing all functions/methods and highlight which data tables and variables are suitable for operation

[lwarode]

• show how to set up docker configuration for R6 object or show in vignette? @FloLimebit what do you think? I would suggest to include it (at least) briefly in the README to be as inclusive as possible for non-technical and rather political (science) target audience

[lwarode]

• mention differentiation of get_ functions: systematic deviation in input arguments (some are just working with specific data tables and deliver a data frame as output, e.g.get_profession_groups, others like get_state are designed to work in dplyr::mutate calls

[lwarode]

• 3 options: show just functions (so far), object & methods, or both