Add citation.

CITATION.cff

@@ -0,0 +1,31 @@
+cff-version: 1.2.0
+message: "If you use this software, please cite it as below."
+authors:
+  - family-names: "Ramos-Carreño"
+    given-names: "Carlos"
+    orcid: "https://orcid.org/0000-0003-2566-7058"
+    affiliation: "Universidad Autónoma de Madrid"
+    email: vnmabus@gmail.com
+title: "dcor: distance correlation and related E-statistics in Python"
+date-released: 2022-03-24
+doi: 10.5281/zenodo.3468124
+url: "https://github.com/vnmabus/dcor"
+license: MIT
+keywords:
+  - "distance correlation"
+  - "distance covariance"
+  - "energy distance"
+  - Python
+identifiers:
+  - description: "This is the collection of archived snapshots of all versions of dcor"
+    type: doi
+    value: 10.5281/zenodo.3468124
+  - description: "This is the archived snapshot of version 0.3 of dcor"
+    type: doi
+    value: 10.5281/zenodo.3468125
+  - description: "This is the archived snapshot of version 0.4 of dcor"
+    type: doi
+    value: 10.5281/zenodo.3779356
+  - description: "This is the archived snapshot of version 0.5 of dcor"
+    type: doi
+    value: 10.5281/zenodo.3996697

README.rst

@@ -1,7 +1,7 @@
 dcor
 ====
 
-|build-status| |docs| |coverage| |pypi| |conda|
+|build-status| |docs| |coverage| |pypi| |conda| |zenodo|
 
 dcor: distance correlation and related E-statistics in Python.
 
@@ -95,4 +95,9 @@ References
 .. |conda| image:: https://anaconda.org/conda-forge/dcor/badges/installer/conda.svg   
     :alt: Available in Conda
     :scale: 100%
-    :target: https://conda.anaconda.org/conda-forge
+    :target: https://conda.anaconda.org/conda-forge
+
+.. |zenodo| image:: https://zenodo.org/badge/DOI/10.5281/zenodo.3468124.svg
+    :alt: Zenodo DOI
+    :scale: 100%
+    :target: https://doi.org/10.5281/zenodo.3468124