Update README.md

README.md

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 # laydient
 Swap filaments throughout am FDM 3D print to create a vertical gradient effect
+
+## usage
+from a command prompt, use ```laydient.py /path/to/source.gcode```
+
+Or place ```/path/to/laydient.py``` in the Post-processing scripts section in PrusaSlicer or maybe other slicers
+
+## how it works
+it searches for ```LAYER_CHANGE``` in the input gcode, counts them, then generates a gradient with that number of steps. Then it searches again and inserts toolchanges into the gcode at the appropriate layers. Finally, it outputs the code as ```input-filename_gradient.gcode``` into the source directory.