Updated README

README.md

@@ -145,4 +145,10 @@ slider_app.py CHART \
     mr250NeuN_c14_s03.ASC \
     mr250NeuN_c14_s03n.jp2 0.0004 \
     --out=mr250_chart_to_neun.reg
-```
\ No newline at end of file
+```
+
+## Coordinate System
+
+SlideR defines:
+- origin is the upper left corner of the image
+- coordinate units are mm