Add vignette for standard user

Goals

• Provide a user manual that enables the standard user (not experienced developer) to use the package.

Steps

1. Read about how to write Vignettes and what a good Vignette is for: http://r-pkgs.had.co.nz/vignettes.html
2. Outline the Vignette, by adding the table of contents in a comment below. Start with the most top-level headlines (e.g. Installation, Creating an empty report, ...) and add more and more detailed / fine-grained sections.
3. As a last step, write the actual content, together with code examples that teach the user to use the package.