Dear Galaxy S6-owning AT&T customer, your time has come. Samsung has edited the phone's support page to show Android 6.0.1. The over-the-air update is going out as version number G920AUCU3CPC2.
The S6 may not be alone. Samsung has also updated the page for the Galaxy S5. That older flagship will get Marshmallow as part of version G900AUCU4DPC1. The when could be another issue. Unlike the S6, the S5 is listed as TBD. Hopefully that translates to soon.