Try improve readme figure quality

Author: chrismainey

README.Rmd

@@ -2,6 +2,7 @@
 output:
   github_document:
     html_preview: false
+    dev: png
 ---
 
 ```{r setup, include=FALSE}
@@ -10,8 +11,10 @@ knitr::opts_chunk$set(
   comment = "#>",
   fig.path = "man/figures/README-",
   out.width = "100%",
-  dev.args = list(png = list(type = "cairo"))
+  dev.args = list(png = list(type = "cairo")),
+  fig.retina=3
 )
+
 ```