Added DOI and citation information (#30)

Author: solliolli

CITATION.cff

@@ -0,0 +1,23 @@
+cff-version: 1.2.0
+message: "If you use this software, please cite it as below."
+authors:
+- family-names: "Oliveira"
+  given-names: "Fabricio"
+  orcid: "https://orcid.org/0000-0003-0300-9337"
+- family-names: "Herrala"
+  given-names: "Olli"
+  orcid: "https://orcid.org/0000-0003-4819-2534"
+- family-names: "Tollander de Balsch"
+  given-names: "Jaan"
+  orcid: "https://orcid.org/0000-0002-4814-5554"
+- family-names: "Hankimaa"
+  given-names: "Helmi"
+  orcid: "https://orcid.org/0000-0001-9531-6080"
+- family-names: "Andelmin"
+  given-names: "Juho"
+  orcid: "https://orcid.org/0000-0003-3874-2029"
+title: "DecisionProgramming.jl"
+version: 1.0.0
+doi: 10.5281/zenodo.5591893
+date-released: 2021-10-22
+url: "https://github.com/gamma-opt/DecisionProgramming.jl"

README.md

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 # DecisionProgramming.jl
 [![Docs Image](https://img.shields.io/badge/docs-latest-blue.svg)](https://gamma-opt.github.io/DecisionProgramming.jl/dev/)
 ![Runtests](https://github.com/gamma-opt/DecisionProgramming.jl/workflows/Runtests/badge.svg)
-
+[![DOI](https://zenodo.org/badge/269314037.svg)](https://zenodo.org/badge/latestdoi/269314037)
 
 ## Description
 `DecisionProgramming.jl` is a [Julia](https://julialang.org/) package for solving multi-stage decision problems under uncertainty, modeled using influence diagrams. Internally, it relies on mathematical optimization. Decision models can be embedded within other optimization models. We designed the package as [JuMP](https://jump.dev/) extension.