Drake: Citation recommendations in readmes and/or webpages

[trashbirdecology]

__Recommendation__
Add the recommended __citation__ to README and/or the package webpage. Also as an FYI, drake readme also doesn't include the instructions for retrieving citation using citation().

__Background__
A colleague (who doesn't really use R) recently approached me asking about how they can easily find the citation for a few R packages, including drake. They said the only instructions they found were "run citation()".

@cboettig et al., I recommend that all rOpenSci packages have recommended citations on their readme/webpages, for our colleagues who do not use or are not familiar enough with R to run citation(). A nice example of doing so is over at osmdata

[wlandau]

A citation is now near the bottom of the README and should appear soon at https://docs.ropensci.org/drake. Thanks.