[Rd] relative path to source files in vignette

[Peter Ruckdeschel]

Gabor Grothendieck schrieb:
> See ?system.file with the package= argument.
>   

Thank you Gabor,

but this refers to the /installed/ package, while we are needing
path information about the not-yet-built source code of the
package during R CMD check / build.

Peter