http://creativecommons.org/licenses/by-sa/4.0/
The Creative Commons Attribution Share-Alike license allows re-distribution and re-use of a licensed work on the conditions that the creator is appropriately credited and that any derivative work is made available under “the same, similar or a compatible license”.
You can use the CC license chooser.
Alternatively, include a link to, or a full copy of the license you use, and something like the following:
This work is licensed under a Creative Commons Attribution Share-Alike [version number] License.