adding note in README from @gonuke helpful clarifying comment

Author: opotowsky

README.md

@@ -3,4 +3,5 @@ Scripts, settings and environment options for better research and analysis
 
 ## tri_heatmap: Heatmaps in matplotlib
 This directory includes a notebook that gives some guidance on plotting 2D
-heatmaps in matplotlib given the existence of X, Y, Z experimental data.  
+heatmaps in matplotlib given the existence of X, Y, Z experimental data.  Note
+this was developed for and is thus ideal for an irregular X-Y grid.