docs: Add CITATION.cff Citation File Format file

* Add Citation File Format file to repo to get repository cite button on GitHub
   - c.f. https://docs.github.com/en/github/creating-cloning-and-archiving-repositories/creating-a-repository-on-github/about-citation-files
* Add CITATION.cff to bump2version control

.bumpversion.cfg

@@ -6,3 +6,5 @@ tag = True
 [bumpversion:file:setup.cfg]
 
 [bumpversion:file:src/recastatlas/config.py]
+
+[bumpversion:file:CITATION.cff]

CITATION.cff

@@ -0,0 +1,45 @@
+cff-version: 1.2.0
+message: "Please cite the following works when using this software."
+type: software
+authors:
+- family-names: "Heinrich"
+  given-names: "Lukas"
+  orcid: "https://orcid.org/0000-0002-4048-7584"
+  affiliation: "Technical University of Munich"
+- family-names: "Feickert"
+  given-names: "Matthew"
+  orcid: "https://orcid.org/0000-0003-4124-7862"
+  affiliation: "University of Wisconsin-Madison"
+title: "recast-atlas: v0.4.0"
+version: 0.4.0
+doi: 10.5281/zenodo.5854896
+repository-code: "https://github.com/recast-hep/recast-atlas/releases/tag/v0.4.0"
+url: "https://github.com/recast-hep/recast-atlas/"
+keywords:
+  - python
+  - physics
+  - RECAST
+  - ATLAS
+  - reinterpretation
+  - analysis reuse
+license: "Apache-2.0"
+references:
+  - type: article
+    authors:
+    - family-names: "Cranmer"
+      given-names: "Kyle"
+      orcid: "https://orcid.org/0000-0002-5769-7094"
+      affiliation: "New York University"
+    - family-names: "Yavin"
+      given-names: "Itay"
+      affiliation: "New York University"
+    title: "RECAST: Extending the Impact of Existing Analyses"
+    doi: 10.1007/JHEP04(2011)038
+    url: "https://doi.org/10.1007/JHEP04(2011)038"
+    year: 2011
+    publisher:
+      name: "Springer"
+    volume: 2011
+    number: 38
+    pages: 2823
+    journal: Journal of High Energy Physics