some minor changes/additions to the readme.rmd

[MartinHinz]

• added setwd as first step,
• added details on permissions for access token

[benmarwick]

Thanks, see my earlier comment on setwd() here: https://github.com/benmarwick/rrtools/pull/10#issuecomment-317485367

It's fine to use setwd() as a throw-away line, perhaps to use when teaching how to do this. But I don't want it to be 'in print' here, because I think it will create or encourage bad habits.

I'll look more into the tokens, thanks!