Mention the core

Author: sverhoeven

_episodes/10-development-setup.md

@@ -111,4 +111,10 @@ confirm that no errors are reported:
 python setup.py test --installation
 ~~~
 
+> ## ESMValCore
+>
+> Most core functionality for the ESMValTool is available in [ESMValCore](https://github.com/ESMValGroup/ESMValCore) Python package.
+> If your contribution in ESMValTool needs changes to ESMValCore please follow [its contribution guide](https://github.com/ESMValGroup/ESMValCore/blob/master/CONTRIBUTING.md).
+{: .callout}
+
 {% include links.md %}