Added aggregation methods to user guide
Added aggregation methods to user guide

noirbizarre/mongoengine,ms5/mongoengine,nathanial/mongoengine,KarimAllah/mongoengine,muhammad-ammar/mongoengine,cogniteev/mongoengine,depop/mongoengine-stripped,Davidrjx/mongoengine,yarneo/mongoengine,MakerReduxCorp/mongoengine,conversocial/mongoengine,jimmyshen/mongoengine,hmarr/mongoengine,elephanter/mongoengine,losintikfos/mongoengine,WingGao/mongoengine,MakerReduxCorp/mongoengine,eli-b/mongoengine,MongoEngine/mongoengine,starsirius/mongoengine,iandd0824/mongoengine,newvem/mongoengine,RussellLuo/mongoengine,ContextLogic/mongoengine,gmacon/mongoengine,MRigal/mongoengine,ConsumerPhysics/mongoengine,larsbutler/mongoengine,blaggacao/mongoengine,nicolascine/mongoengine,ContextLogic/mongoengine,Multiposting/mongoengine,DavidBord/mongoengine,khallaghi/mongoengine,Catstyle/mongoengine,ProgressivePlanning/mongoengine,touilleMan/mongoengine,prijindal/mongoengine,sourcelair/mongoengine,aszwemin/mongoengine,cogniteev/mongoengine,Davidrjx/mongoengine,alex/mongoengine,reallistic/mongoengine,colinhowe/mongoengine,9nix00/mongoengine,closeio/mongoengine,gavincabbage/mongoengine