add code of conduct to the doc

Author: toin0u

README.md

@@ -42,6 +42,13 @@ Credits
 * [All contributors](https://github.com/geocoder-php/BazingaGeocoderBundle/contributors)
 
 
+Contributor Code of Conduct
+---------------------------
+
+Please note that this project is released with a Contributor Code of Conduct.
+By participating in this project you agree to abide by its terms.
+
+
 License
 -------