Add a few notes about future mgo.v3 changes.
Add a few notes about future mgo.v3 changes.

go-mgo/travis-test,deafgoat/mgo,DroiTaipei/mgo,DroiTaipei/mgo,gabrielrussell/mgo,jackspirou/mgo,jackspirou/mgo,jackspirou/mgo,Yottaa/mgo.v2,deafgoat/mgo,jackspirou/mgo,hexadecy/mgo,gabrielrussell/mgo,gabrielrussell/mgo,go-mgo/travis-test,hexadecy/mgo,deafgoat/mgo,go-mgo/travis-test,Yottaa/mgo.v2,gabrielrussell/mgo,Yottaa/mgo.v2,DroiTaipei/mgo,Yottaa/mgo.v2,hexadecy/mgo,deafgoat/mgo,go-mgo/travis-test,hexadecy/mgo,DroiTaipei/mgo