S6 AT&T will be updating in the end of the month, so judging from the history, I don't think we can get M before June.
Even if they update to M, I will probably waiting a few more months to see if there is any root on 5.1.1, since I didn't see any feature I like from 6.0 yet. There are already a possible method for S6 AT&T here:
[ROOT] AT&T S6 (g920a) 5.1.1 - Tethered … | AT&T Samsung Galaxy S6
This root method is for the plain S6 (g920a), but we have the Active (G890a). The processes are not fungible among models (please let me know if I missed something and they are!).
Since buying the S6 Active it has been the longest time that I have ever gone unrooted on an Android phone (about 8 months so far!). I, too, would jump at a chance to root this phone!
Up until now, I've (not so) patiently accepted the s-l-o-w ATT updates and used other methods to freeze the bloat as much as is practicable.
I think we may see Marshmallow, but probably about 60-90 days after the ATT S6
and S6 Edge get it. ;-(
N, on the other hand, is something that I may or may not see by the time my S6 Active phone is obsolete in a couple of years! -This is ATT after all.