Update Density coloured scatterplots with ggplot2/README.md

Density coloured scatterplots with ggplot2/README.md

+# Density coloured scatter plots to avoid overplotting
+
+Any time.
+
+Running the included example from the R script leads to this image:
+
+![xyplot](<./xyplot_density.png> "X, Y scatterplot with density information")
+
+The result will need further adjustments, in Inkscape.