Add note about MongoDB secret backend to changelog
Add note about MongoDB secret backend to changelog

lirao/vault,peteryhwong/vault,deverton/hashicorp-vault,MonsantoCo/vault,jgcallero/vault,rnaveiras/vault,lirao/vault,naunga/vault,hashicorp/vault,Caiyeon/vault,Aloomaio/vault,rnaveiras/vault,hashicorp/vault,MonsantoCo/vault,joelthompson/vault,mhurne/vault,memory/vault,cimarron-pistoncloud/vault,Aloomaio/vault,quixoten/vault,joelthompson/vault,hartsock/vault,Caiyeon/vault,mgk/vault,chrishoffman/vault,LolalyLuo/vault,tallpauley/vault,deverton/hashicorp-vault,rnaveiras/vault,peteryhwong/vault,hartsock/vault,joelthompson/vault,mgk/vault,rnaveiras/vault,mymercurialsky/vault,jmhardison/vault,quixoten/vault,wjam/vault,mymercurialsky/vault,martin-ducar-gd/vault,jam-pot/vault,doodles526/vault,hashicorp/vault,kingland/vault,joelthompson/vault,naunga/vault,skippy/vault,quixoten/vault,jmhardison/vault,kingland/vault,jam-pot/vault,jmhardison/vault,lemondrank/vault,hartsock/vault,wjam/vault,michaelansel/vault,ryon/vault,michaelansel/vault,doodles526/vault,rnaveiras/vault,kingland/vault,mgk/vault,hashicorp/vault,quixoten/vault,Banno/vault,kingland/vault,cimarron-pistoncloud/vault,Aloomaio/vault,hashicorp/vault,LolalyLuo/vault,ryon/vault,skippy/vault,hartsock/vault,lemondrank/vault,hashicorp/vault,mhurne/vault,jmhardison/vault,jmhardison/vault,martin-ducar-gd/vault,kingland/vault,tallpauley/vault,Aloomaio/vault,danielsomerfield/vault,hartsock/vault,mgk/vault,chrishoffman/vault,danielsomerfield/vault,memory/vault,jgcallero/vault,Banno/vault