Enhancement: Add notes where functions are located in the source

[ff6347]

While skimming through the source and looking for the place where the ENV variables are included it was hard for me to find out where functions are coming from (since they all seem to exist in a global space).

I think it would be a good addition to use a package like the one below to make it easier to see where functions are coming from. (Just my 50 cents)

• https://github.com/smbache/import
• https://cran.r-project.org/web/packages/import/vignettes/import.html

re #15