Added citation file

Author: mtomasini

CITATION.cff

@@ -0,0 +1,9 @@
+cff-version: 1.2.0
+message: "If you use this software, please cite it as below."
+authors:
+  - family-names: Tomasini
+    given-names: Matteo
+    orcid: https://orcid.org/0000-0003-2776-9998
+title: "MetaPypulation"
+version: 0.0.1
+url: "https://github.com/mtomasini/MetapopulationsPython"