add license notes of neural networks

books/free-programming-books-subjects.md

@@ -403,7 +403,7 @@ Books that cover a specific programming language can be found in the [BY PROGRAM
 ### Machine Learning
 
 * [A Brief Introduction to Machine Learning for Engineers](https://arxiv.org/pdf/1709.02840.pdf) - Osvaldo Simeone (PDF)
-* [A Brief Introduction to Neural Networks](https://www.dkriesel.com/en/science/neural_networks)
+* [A Brief Introduction to Neural Networks](https://www.dkriesel.com/en/science/neural_networks) (CC BY-ND)
 * [A Comprehensive Guide to Machine Learning](https://www.eecs189.org/static/resources/comprehensive-guide.pdf) - Soroush Nasiriany, Garrett Thomas, William Wang, Alex Yang (PDF)
 * [A Course in Machine Learning](http://ciml.info/dl/v0_9/ciml-v0_9-all.pdf) (PDF)
 * [A First Encounter with Machine Learning](https://web.archive.org/web/20210420163002/https://www.ics.uci.edu/~welling/teaching/ICS273Afall11/IntroMLBook.pdf) - Max Welling (PDF) *(:card_file_box: archived)*