Update README.md with an example image

Author: NicholasMamo

README.md

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 A Python library that builds on matplotlib, providing new types of visualizations to help you explore your data and explain it better.
 
+![Example time series](https://raw.githubusercontent.com/NicholasMamo/multiplex-plot/master/examples/exports/3-time-series.png)
+
 ## Getting Started
 
 These instructions will get you a copy of the project up and running.