Update M428.md (MarlinFirmware#150)

Lukelectro · thinkyhead · commit eb794b58e973 · 2018-04-19T21:17:46.000-05:00
diff --git a/_gcode/M428.md b/_gcode/M428.md
@@ -14,6 +14,7 @@ codes:
 long: |
   Use `M428` to set a persistent offset to the native home position and coordinate space by assigning the current position as the native home position. See the example below.
 
+  - The current position must be within 2cm from 0 or an endstop.
   - The current position is set to the native home position.
   - Any previous position shift from `G92` is cleared.
   - The home offset is persistent — added to the current position until changed.
