Yep credit is given. I'm not sure about the asking though, the License says you can redistro and modify. However it would IMHO be best to contact the author on a practical level they may have knowledge/experience that you don't have.
Say for instance I create a historical map for a historical battle, unfortunately my sources for the OB's are somewhat lacking. It would be best use of time if someone else with that knowledge could use my map to improve on my scenario. Could be years after I loose interest in the game! The community benefits and I won't come along at a later date demanding that 12 different versions all using my map get taken down.
Note; It is as BFC say - your work, you can restrict how you like - I was mearly pointing out a very cheep/successful way of developing. There are others that the Linux community uses.
Dave