Add 3DFabXYZ Migbot example config (#12775)

Co-Authored-By: Alain Martel <alain74martel@gmail.com>

Marlin/src/config/examples/3DFabXYZ/Migbot/Readme.md

@@ -0,0 +1,3 @@
+Personalized config files for the "Electron"\"Tevo 3D" i3 6th gen
+https://www.3dprintersbay.com/electron3d-reprap-prusa-i3-kit
+https://reprap.org/wiki/Migbot_Prusa_i3