added another note for existing users

readme.textile

@@ -10,4 +10,6 @@ Grbl includes full acceleration management with look ahead. That means the contr
 
 *Important note for Atmega 168 users:* Grbl used to be compatible with both the older Ardunios running atmega 168 and the newer with 328p. This had to go, as I was unable to fit the acceleration management into the 16k code space of the 168. If you want to run Grbl on an 168 I am still maintaining Grbl 0.51 "in the branch called 'v0_51'":https://github.com/simen/grbl/tree/v0_51.
 
+*Note for users upgrading from 0.51 to 0.6:* The new version has new and improved default pin-out. If nothing works when you upgrade, that is because the pulse trains are coming from the wrong pins. This is a simple matter of editing config.h – the whole legacy pin assignment is there for you to uncomment.
+
 _The project was initially inspired by the Arduino GCode Interpreter by Mike Ellery_