[fix carpentries/styles#300] add CODE_OF_CONDUCT.md to root

Author: fmichonneau

CODE_OF_CONDUCT.md

@@ -0,0 +1,11 @@
+---
+layout: page
+title: "Contributor Code of Conduct"
+---
+As contributors and maintainers of this project,
+we pledge to follow the [Carpentry Code of Conduct][coc].
+
+Instances of abusive, harassing, or otherwise unacceptable behavior
+may be reported by following our [reporting guidelines][coc-reporting].
+
+{% include links.md %}