Update README.md

README.md

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 ![1MHz](doc/SQ-1MHz.jpg)  
 
 Above image shows 1MHz square waves on both output channels. It must be noted that the R value in the ladder networks is 0.5k in channel A and 1k in channel B, nicely showing the low-pass effect. 
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 ![Proto-schematic](doc/Proto-schematic.gif) 
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+# Copyright notice
+**The code and electronic designs as well as the implementations presented in this repository can be copied and modified freely, for non-commercial use.
+Use for commercial purposes is allowed as well, as long as a reference to this repository is included in the product.**