Update README.md

[ci skip]

Author: jgabry

README.md

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 [![codecov](https://codecov.io/gh/stan-dev/bayesplot/branch/master/graph/badge.svg)](https://codecov.io/gh/stan-dev/bayesplot)
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+### Overview
+
 **bayesplot** is an R package providing an extensive library of plotting
 functions for use after fitting Bayesian models (typically with MCMC). 
 The plots created by **bayesplot** are ggplot objects, which means that after