Update introduction.Rmd

Fix minor typos
Merge pull request #307 from cwickham/master

Update introduction.Rmd
Add some useful db resources
Fix join bugs. #310
Don't try and print empty results. Closes #310
fix minor typos in window-functions.Rmd
Merge pull request #312 from justmarkham/master

fix minor typos in window-functions.Rmd
Merge branch 'master' of https://github.com/hadley/dplyr
MonetDB connector documentation update
MonetDB connector: roxygen run