Added citation file (#1082)

Closes #1081 with the suggested `CITATION.cff` file. I've enabled Zenodo
integration and generated [DOIs](https://zenodo.org/record/7737486) for
all releases of the repo. This can be updated in the future with
something more formal, such as a paper citation.

CITATION.cff

@@ -0,0 +1,27 @@
+cff-version: 1.2.0
+message: "If you use this software, please cite it as below."
+authors:
+- family-names: "Kanazawa"
+  given-names: "Naoki"
+  orcid: "https://orcid.org/0000-0002-4192-5558"
+- family-names: "Egger"
+  given-names: "Daniel J."
+  orcid: "https://orcid.org/0000-0002-5523-9807"
+- family-names: "Ben-Haim"
+  given-names: "Yael"
+- family-names: "Zhang"
+  given-names: "Helena"
+  orcid: "https://orcid.org/0000-0002-7813-7133"
+- family-names: "Shanks"
+  given-names: "Will E."
+  orcid: "https://orcid.org/0000-0002-5045-8808"
+- family-names: "Aleksandrowicz"
+  given-names: "Gadi"
+- family-names: "Wood"
+  given-names: "Christopher J."
+  orcid: "https://orcid.org/0000-0001-7606-7349"
+title: "Qiskit Experiments"
+version: 0.4.0
+doi: 10.5281/zenodo.7737486
+date-released: 2022-08-16
+url: "https://github.com/Qiskit/qiskit-experiments"