Add License Info for Learning C++ eBook (#11657)

* Add GPL-3.0-or-later license to 'Matters Computational' book entry

* Update free-programming-books-langs.md with new resources and licenses

books/free-programming-books-langs.md

@@ -542,7 +542,7 @@ Books on general-purpose programming that don't focus on a specific language are
 * [Joint Strike Fighter, C++ Coding Standards](http://www.stroustrup.com/JSF-AV-rules.pdf) - Bjarne Stroustrup (PDF)
 * [Learn C++ Programming Language](http://www.tutorialspoint.com/cplusplus/cpp_tutorial.pdf) - Tutorials Point (PDF)
 * [LearnCpp.com](https://www.learncpp.com) (HTML)
-* [Learning C++ eBook](https://riptutorial.com/Download/cplusplus.pdf) - Compiled from StackOverflow Documentation (PDF)
+* [Learning C++ eBook](https://riptutorial.com/Download/cplusplus.pdf) - Compiled from StackOverflow Documentation (PDF) (CC BY-SA) 
 * [Matters Computational: Ideas, Algorithms, Source Code](http://www.jjj.de/fxt/fxtbook.pdf) - Jorg Arndt (PDF)
 * [Modern C++ Tutorial: C++11/14/17/20 On the Fly](https://www.changkun.de/modern-cpp/pdf/modern-cpp-tutorial-en-us.pdf) - Changkun Ou (PDF) (CC BY-NC-ND)
 * [More C++ Idioms](https://en.wikibooks.org/wiki/More_C%2B%2B_Idioms) - Sumant Tambe, et al. (WikiBooks)