Mention the Code of Conduct in CONTRIBUTING.md

CONTRIBUTING.md

@@ -1,6 +1,9 @@
-## Contributor license agreement
+## Contributor License Agreement
 By contributing you agree to the [LICENSE](https://github.com/vhf/free-programming-books/blob/master/LICENSE) of this repository.
 
+## Contributor Code of Conduct
+By contributing you agree to respect the [Code of Conduct](https://github.com/vhf/free-programming-books/blob/master/CODE_OF_CONDUCT.md) of this repository.
+
 ## In a nutshell
 1. "An link to easily download a book" is not alway a link to a *free* book. Please only contribute free content. Make sure it's free.
 2. You don't have to know git: if you found something of interest which is *not already in this repo*, please open an issue with your links propositions.