Add note about ``tmux -2``
Add note about ``tmux -2``

Closes #847

magus424/powerline,kenrachynski/powerline,prvnkumar/powerline,seanfisk/powerline,bezhermoso/powerline,bartvm/powerline,junix/powerline,xfumihiro/powerline,areteix/powerline,cyrixhero/powerline,bartvm/powerline,areteix/powerline,IvanAli/powerline,IvanAli/powerline,S0lll0s/powerline,IvanAli/powerline,darac/powerline,DoctorJellyface/powerline,areteix/powerline,magus424/powerline,xxxhycl2010/powerline,cyrixhero/powerline,EricSB/powerline,blindFS/powerline,lukw00/powerline,firebitsbr/powerline,DoctorJellyface/powerline,Liangjianghao/powerline,bezhermoso/powerline,blindFS/powerline,bartvm/powerline,dragon788/powerline,wfscheper/powerline,xxxhycl2010/powerline,bezhermoso/powerline,kenrachynski/powerline,s0undt3ch/powerline,prvnkumar/powerline,Luffin/powerline,s0undt3ch/powerline,xxxhycl2010/powerline,wfscheper/powerline,junix/powerline,xfumihiro/powerline,Luffin/powerline,darac/powerline,S0lll0s/powerline,EricSB/powerline,russellb/powerline,Liangjianghao/powerline,S0lll0s/powerline,darac/powerline,Luffin/powerline,s0undt3ch/powerline,Liangjianghao/powerline,junix/powerline,seanfisk/powerline,QuLogic/powerline,cyrixhero/powerline,firebitsbr/powerline,QuLogic/powerline,kenrachynski/powerline,lukw00/powerline,russellb/powerline,xfumihiro/powerline,wfscheper/powerline,blindFS/powerline,dragon788/powerline,prvnkumar/powerline,DoctorJellyface/powerline,seanfisk/powerline,firebitsbr/powerline,lukw00/powerline,EricSB/powerline,QuLogic/powerline,dragon788/powerline,russellb/powerline,magus424/powerline