lme4/lme4 — pull request #242 by drammock (2014-09-03 08:11:08+00:00)
Title: DOC: troubleshooting advice for error about factors with <2 levels

Following up on @bbolker's suggestion [here](https://stat.ethz.ch/pipermail/r-sig-mixed-models/2014q3/022596.html) to add the `Contrasts can be applied only to factors with 2 or more levels` error to the troubleshooting docs.