Update news about ggplot2 themes

NEWS.md

-# xaringanthemer 0.3.0
+# xaringanthemer 0.3.0 (dev)
+
+* Adds `theme_xaringan()`, a ggplot2 theme that automatically uses the colors
+  and font styles of your xaringanthemer style. Also adds 
+  `scale_xaringan_color()` and `scale_xaringan_fill()` functions that use the
+  primary colors for the color or fill aesthetics. These theme functions
+  currently do not work when knitr caching is enabled and require that the
+  xaringanthemer style functions are called within your xaringan slides Rmd file.
 
 * Renamed all xaringanthemer functions that create CSS styles to use the
   prefix "`style_`". For example, `mono_light()` is now `style_mono_light()`.