Update README.md

Author: rwpogge

README.md

@@ -26,7 +26,8 @@ and new explorations rather than being frozen into print.
 All notebooks were developed in Python 3 using the [Anaconda Python distribution](https://www.anaconda.com). 
 
 Required Python packages are numpy, scipy, pandas, matplotlib, and astropy, all of which are part of the
-Anaconda Python distribution.
+Anaconda Python distribution. Because of significant changes in matplotlib starting with version 3.8
+we strongly recommend updating to the latest version of matplotlib if you haven't already.
 
 LaTeX is required to plot math symbols in the notebooks.