Update README.txt

DiffractionGratings/README.txt

 Parametric model of diffraction gratings
 
-Quick start
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+Quick start 2D
+--------------
 
 Open `grating2D.pro' with Gmsh.
 
+Quick start 3D
+--------------
+
+To run the test cases, type in the command line :
+'gmsh grating3D.pro -setstring test_case hole'
+
 Additional info
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