DOC: binary colorscheme expl in readme

README.md

@@ -69,6 +69,11 @@ fig.set_size_inches(8, 8)
 
 ![Output dotplot from the above example](https://github.com/fedarko/wotplot/raw/main/docs/img/ecoli_example_dotplot.png)
 
+When visualizing a binary matrix (and/or using `viz_spy()`), the default
+colorscheme uses black cells (⬛) to indicate matches (forward,
+reverse-complementary, or palindromic)
+and white cells (⬜) to indicate no matches.
+
 ## More detailed tutorial
 
 Please see [this Jupyter Notebook](https://nbviewer.org/github/fedarko/wotplot/blob/main/docs/Tutorial.ipynb).