Update read me. Added comment about products that have a patent.

README.md

@@ -1,6 +1,8 @@
 ==========================
 Marlin 3D Printer Firmware
 ==========================
+Marlin has a GPL license because I believe in open development.
+Please do not use this code in products (3D printers, CNC etc) that are closed source or are crippled by a patent.
 
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