added citation for how it works article

CITATION.md

@@ -2,9 +2,17 @@
 
 If you use this project in publications, please cite the following:
 
-    @misc{projection-pursuit,
-          title={Projection pursuit software}, 
-          author={Pavel Komarov}, 
-          url={http://www.github.com/pavelkomarov/projection-pursuit}, 
-          note={Version 1.1},
-          year={2024}}
+	@misc{projection-pursuit,
+		title={Projection pursuit software}, 
+		author={Pavel Komarov}, 
+		url={http://www.github.com/pavelkomarov/projection-pursuit}, 
+		note={Version 1.1},
+		year={2024}}
+
+If you wish to cite the mathematical explanation, use:
+
+	@article{projection-pursuit-math
+		title={The Gory Details},
+		author={Pavel Komarov},
+		url={https://pavelkomarov.com/projection-pursuit/math.pdf},
+		year={2018}}