doc: add coordinate scaling caveat to README

Author: jvkersch

README.md

@@ -119,6 +119,13 @@ you want to have it added here, please open an issue.
 Technical Notes
 -------
 
+**Coordinate scaling:** Concorde computes distances using integer arithmetic.
+If your coordinates are small (e.g. in [0, 1]), distances between points will
+be rounded to 0, leading to incorrect results or crashes. Scale your
+coordinates up before passing them to the solver — for example, multiply by
+1e6. Similarly, very large coordinates (above 1e7) may cause integer overflow.
+See `examples/truncation_demo.py` for a demonstration.
+
 PyConcorde needs Concorde and QSOpt. Downloading and building these packages
 should happen automatically on Linux/Mac OS, but please file an issue if you
 experience any trouble during this step.