Link to code of conduct from CONTRIBUTING.md

CONTRIBUTING.md

 # How to contribute
 
+## Code of conduct
+
+We follow the [Contributor Covenant](https://www.contributor-covenant.org) code
+of conduct, please see the [CODE_OF_CONDUCT.md](./CODE_OF_CONDUCT.md) document
+for details.
+
 ## Preparing the development environment
 
 Make sure you have development dependencies installed in your development environment.