Update docstring to explain relative paths in arcpy

generate_heatmaps.py

@@ -2,6 +2,11 @@
 
 This script must be run from the version of python installed with ArcGIS, e.g.
 C:/Python27/ArcGIS10.4/python generate_heatmaps.py
+
+Note: arcpy does not play nicely with relative paths. This is because arcpy
+changes the python working directory when an mxd is loaded, and during many
+mapping operations. Relative paths may be used as inputs to this script, but
+they will be converted to absolute paths internally.
 """
 
 import os