Added "C++ Core Guidelines" (#2069)

Modern authoritative guidelines for writing C++ code (the initial primary authors and maintainers are Bjarne Stroustrup and Herb Sutter and the guidelines so far were developed with contributions from experts at CERN, Microsoft, Morgan Stanley, and several other organizations).

free-programming-books.md

@@ -849,6 +849,7 @@
 
 * [C++ Annotations](https://fbb-git.github.io/cppannotations/)
 * [C++ Cookbook](http://staff.ppu.edu/dkhalid/O'Reilly%20-%20C++%20Cookbook%20%282007%29.pdf) (PDF)
+* [C++ Core Guidelines](https://github.com/isocpp/CppCoreGuidelines/blob/master/CppCoreGuidelines.md) - Editors: Bjarne Stroustrup, Herb Sutter
 * [C++ GUI Programming With Qt 3](http://www.computer-books.us/cpp_0010.php)
 * [C++ Succinctly, Syncfusion](https://www.syncfusion.com/resources/techportal/ebooks/cplusplus) (PDF, Kindle) *(Just fill the fields with any values)*
 * [C++ Tricks](http://www.bordoon.com/cplusplus/book_wrapper.html)