removed reference to roxygen3

**Only accept this change if** as I suspect, the roxygen3 features described here have merged into roxygen2

The [roxygen3 README](https://github.com/hadley/roxygen3/blob/master/README.md) indicates development has stopped ... the deleted text was added oct 2012 (e6ebfd22) and roxygen3 was deprecated 11/2013; so it seems that this note is no longer relevant.