Updated upgrade docs.  Reported issues with older pymongo and sharding
Updated upgrade docs.  Reported issues with older pymongo and sharding

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