add wilkinson notation reference

Author: drbenvincent

docs/source/references.bib

@@ -40,3 +40,14 @@ @article{carpenter2009effect
   year={2009},
   publisher={American Economic Association}
 }
+
+@article{wilkinson1973symbolic,
+  title={Symbolic description of factorial models for analysis of variance},
+  author={Wilkinson, GN and Rogers, CE},
+  journal={Journal of the Royal Statistical Society Series C: Applied Statistics},
+  volume={22},
+  number={3},
+  pages={392--399},
+  year={1973},
+  publisher={Oxford University Press}
+}