doc: Note the build dependency on cairosvg

Author: rsmarples

README.md

@@ -28,6 +28,9 @@ For each `--with` there is a `--without` and for each `--enable` a `--disable`.
 If each part is not specified then the configure will test the system
 for the needed libraries to build and install it.
 
+(cariosvg)[https://cairosvg.org/) is used to build the icons from the svg source.
+It's not a runtime dependency.
+
 ### Notifications
 
 Notifications are dependant on the chosen platform.