Add citation file

CITATION.cff

@@ -0,0 +1,21 @@
+cff-version: 1.2.0
+message: "If you use this software, please cite it as below."
+authors:
+- family-names: Fabisch
+  given-names: Alexander
+title: "gmr"
+url: https://github.com/AlexanderFabisch/gmr
+doi: 10.5281/zenodo.4449631
+preferred-citation:
+    type: article
+    authors:
+    - family-names: Fabisch
+      given-names: Alexander
+    title: "gmr: Gaussian Mixture Regression"
+    doi: 10.21105/joss.03054
+    journal: Journal of Open Source Software
+    start: 3054
+    issue: 62
+    volume: 6
+    month: 6
+    year: 2021