How do I create a game that can support mod?

In order to supply the editor with your game you need a license by Crytek. Contact them for details. You develop the game the normal way. Code in what changes you need in the editor and rebuild the code if necessary. Some SDKs might require a license to use, not sure.

What do you mean by In order to supply the editor with your game you need a license by Crytek. Contact them for details and Code in what changes you need in the editor and rebuild the code if necessary. Some SDKs might require a license to use?

Exactly what it means, you cant distribute your game with the CRYENGINE Editor without a special license from Crytek. Contact them about it and ask about the details.

https://www.cryengine.com/support

What type of license from Crytek?

one that would allow you to include the editor with your build. You need to contact Crytek about that. I cannot help you there.