Added the Code of Conduct to Symfony Docs repo

Author: javiereguiluz

.github/CODE_OF_CONDUCT.md

@@ -0,0 +1,12 @@
+Code of Conduct
+===============
+
+This project follows a [Code of Conduct][code_of_conduct] in order to ensure an
+open and welcoming environment. Please read the full text for understanding the
+accepted and unaccepted behavior.
+
+Please read also the [reporting guidelines][guidelines], in case you encountered
+or witnessed any misbehavior.
+
+[code_of_conduct]: https://symfony.com/doc/current/contributing/code_of_conduct/code_of_conduct.html
+[guidelines]: https://symfony.com/doc/current/contributing/code_of_conduct/reporting_guidelines.html