mutate_if dplyr function in ReadMe

[wschwarzmann]

The mutate_if dplyr function used as an example in the ReadMe has been superseded by a new syntax. It should still work in most cases, but it might be good to update at some point.

[stufield]

@wschwarzmann , this will likely be taken care of during the next release as part of general upgrades. Thanks for bringing it to my attention!