Custom Game: Can people make maps and mods?

[mktwo]

Well, one thing I recall seeing is that you aren't allowed to include an editor with your game, so I think mapping's out of the question.

[Sanch3z]

I'm working on a game and would like for people, not on the dev team, to be able to create custom maps and mods for.

Is this possible? Yes I did search for an answer and I found other threads asking this but there was never a good answer given.

Thanks