Add reference to origin of Z axis being the floor (#2614)

source/docs/software/basic-programming/coordinate-system.rst

@@ -8,7 +8,7 @@ It is important to understand the basics of the coordinate system used throughou
 WPILib coordinate system
 ------------------------
 
-In most cases, WPILib uses the NWU axes convention (North-West-Up as external reference in the world frame.) In the NWU axes convention, where the positive X axis points ahead, the positive Y axis points left, and the positive Z axis points up. When viewed with each positive axis pointing toward you, counter-clockwise (CCW) is a positive value and clockwise (CW) is a negative value.
+In most cases, WPILib uses the NWU axes convention (North-West-Up as external reference in the world frame.) In the NWU axes convention, where the positive X axis points ahead, the positive Y axis points left, and the positive Z axis points up referenced from the floor. When viewed with each positive axis pointing toward you, counter-clockwise (CCW) is a positive value and clockwise (CW) is a negative value.
 
 .. figure:: images/coordinate-system/robot-3d.svg
    :scale: 200