add code citation

Author: mileslucas

CITATION.bib

@@ -1,3 +1,21 @@
+# zenodo DOI citation
+@software{miles_lucas_2021_5808196,
+  author       = {Miles Lucas and
+                  Saranjeet Kaur and
+                  Tor Erlend Fjelde and
+                  Alexander Plavin and
+                  Hong Ge and
+                  Cameron Pfiffer},
+  title        = {TuringLang/NestedSamplers.jl: v0.8.1},
+  month        = dec,
+  year         = 2021,
+  publisher    = {Zenodo},
+  version      = {v0.8.1},
+  doi          = {10.5281/zenodo.5808196},
+  url          = {https://doi.org/10.5281/zenodo.5808196}
+}
+
+## References
 # original nested sampling algorithm
 @article{skillingNestedSampling2004,
 	title = {Nested {Sampling}},