Added subscription service documentation and extended the existing text a bit
Added subscription service documentation and extended the existing text a bit


Former-commit-id: 10ac3563c3a483026d92f44d49a309140b2bd493
MassTransit/MassTransit,jacobpovar/MassTransit,phatboyg/MassTransit,jacobpovar/MassTransit,MassTransit/MassTransit,SanSYS/MassTransit,phatboyg/MassTransit,SanSYS/MassTransit