added license notes to 2 books (#11442)

books/free-programming-books-subjects.md

@@ -233,8 +233,8 @@ Books that cover a specific programming language can be found in the [BY PROGRAM
 * [Computational Thinking](https://www.cs.cmu.edu/~15110-s13/Wing06-ct.pdf) - Jeannette Wing, Carnegie-Mellon University (PDF)
 * [Computer Science Class XI](https://cbseacademic.nic.in/web_material/doc/cs/1_Computer-Science-Python-Book-Class-XI.pdf) - CBSE (PDF)
 * [Computer Science Class XII](https://cbseacademic.nic.in/web_material/doc/cs/2_Computer_Science_Python_ClassXII.pdf) - CBSE (PDF)
-* [Computer Science I](https://cse.unl.edu/~cbourke/ComputerScienceOne.pdf) - Chris Bourke (PDF)
-* [Computer Science II](https://cse.unl.edu/~cbourke/ComputerScienceTwo.pdf) - Chris Bourke (PDF)
+* [Computer Science I](https://cse.unl.edu/~cbourke/ComputerScienceOne.pdf) - Chris Bourke (PDF) (CC BY-SA) 
+* [Computer Science II](https://cse.unl.edu/~cbourke/ComputerScienceTwo.pdf) - Chris Bourke (PDF) (CC BY-SA)
 * [CS Principles: Big Ideas in Programming](https://www.openbookproject.net/books/StudentCSP/) - Mark Guzdial, Barbara Ericson (HTML)
 * [Pull Requests and Code Review](https://scs.tl/book-pr) – Sebastien Castiel
 * [What to Look for in a Code Review](https://leanpub.com/whattolookforinacodereview) - Trisha Gee (HTML, PDF, EPUB, Kindle) *(Leanpub account or valid email requested)*