Update README with DOI

Author: RLumSK

README.Rmd

@@ -21,6 +21,7 @@ The **R** package RLumCarlo provides a collection of R functions modelling lumin
 
 [![Project Status: Concept](https://www.repostatus.org/badges/latest/active.svg)](https://www.repostatus.org/#active)
 [![CRAN](https://www.r-pkg.org/badges/version/RLumCarlo)](https://CRAN.R-project.org/package=RLumCarlo)
+[![DOI](https://zenodo.org/badge/82188344.svg)](https://zenodo.org/badge/latestdoi/82188344)
 [![Downloads](https://cranlogs.r-pkg.org/badges/grand-total/RLumCarlo)](https://www.r-pkg.org/pkg/RLumCarlo)
 [![Build Status](https://travis-ci.org/R-Lum/RLumCarlo.svg?branch=master)](https://travis-ci.org/R-Lum/RLumCarlo)
 [![Build status](https://ci.appveyor.com/api/projects/status/yxsmwa86jajhtesn?svg=true)](https://ci.appveyor.com/project/RLumSK/rlumcarlo)