doc: Add historic 0.21.0 release notes
doc: Add historic 0.21.0 release notes

JeremyRubin/bitcoin,anditto/bitcoin,instagibbs/bitcoin,Sjors/bitcoin,lateminer/bitcoin,sstone/bitcoin,jnewbery/bitcoin,mm-s/bitcoin,n1bor/bitcoin,particl/particl-core,AkioNak/bitcoin,sipsorcery/bitcoin,practicalswift/bitcoin,jambolo/bitcoin,fujicoin/fujicoin,bitcoinsSG/bitcoin,andreaskern/bitcoin,sstone/bitcoin,MarcoFalke/bitcoin,achow101/bitcoin,qtumproject/qtum,namecoin/namecoin-core,lateminer/bitcoin,pstratem/bitcoin,rnicoll/bitcoin,domob1812/namecore,practicalswift/bitcoin,achow101/bitcoin,MeshCollider/bitcoin,jamesob/bitcoin,sstone/bitcoin,kallewoof/bitcoin,ajtowns/bitcoin,lateminer/bitcoin,bitcoin/bitcoin,instagibbs/bitcoin,mm-s/bitcoin,cdecker/bitcoin,yenliangl/bitcoin,GroestlCoin/GroestlCoin,pstratem/bitcoin,GroestlCoin/bitcoin,practicalswift/bitcoin,EthanHeilman/bitcoin,ElementsProject/elements,yenliangl/bitcoin,fanquake/bitcoin,n1bor/bitcoin,MarcoFalke/bitcoin,sipsorcery/bitcoin,namecoin/namecore,jamesob/bitcoin,instagibbs/bitcoin,ajtowns/bitcoin,Xekyo/bitcoin,jlopp/statoshi,jlopp/statoshi,ElementsProject/elements,fujicoin/fujicoin,pstratem/bitcoin,domob1812/namecore,bitcoinsSG/bitcoin,bitcoin/bitcoin,apoelstra/bitcoin,JeremyRubin/bitcoin,qtumproject/qtum,pstratem/bitcoin,Xekyo/bitcoin,bitcoin/bitcoin,pataquets/namecoin-core,anditto/bitcoin,EthanHeilman/bitcoin,MarcoFalke/bitcoin,fanquake/bitcoin,pataquets/namecoin-core,fanquake/bitcoin,pstratem/bitcoin,GroestlCoin/GroestlCoin,prusnak/bitcoin,AkioNak/bitcoin,MeshCollider/bitcoin,Sjors/bitcoin,domob1812/namecore,rnicoll/bitcoin,n1bor/bitcoin,Sjors/bitcoin,jnewbery/bitcoin,kallewoof/bitcoin,bitcoin/bitcoin,anditto/bitcoin,bitcoinknots/bitcoin,mruddy/bitcoin,n1bor/bitcoin,sstone/bitcoin,practicalswift/bitcoin,JeremyRubin/bitcoin,instagibbs/bitcoin,dscotese/bitcoin,jlopp/statoshi,mruddy/bitcoin,domob1812/bitcoin,GroestlCoin/GroestlCoin,sstone/bitcoin,kallewoof/bitcoin,ElementsProject/elements,jambolo/bitcoin,yenliangl/bitcoin,bitcoinknots/bitcoin,qtumproject/qtum,cdecker/bitcoin,instagibbs/bitcoin,fanquake/bitcoin,mruddy/bitcoin,particl/particl-core,AkioNak/bitcoin,prusnak/bitcoin,particl/particl-core,namecoin/namecore,dscotese/bitcoin,namecoin/namecoin-core,GroestlCoin/GroestlCoin,ajtowns/bitcoin,domob1812/bitcoin,sipsorcery/bitcoin,EthanHeilman/bitcoin,jnewbery/bitcoin,yenliangl/bitcoin,bitcoinsSG/bitcoin,achow101/bitcoin,dscotese/bitcoin,andreaskern/bitcoin,jnewbery/bitcoin,instagibbs/bitcoin,particl/particl-core,particl/particl-core,Sjors/bitcoin,domob1812/bitcoin,pataquets/namecoin-core,jlopp/statoshi,domob1812/bitcoin,jambolo/bitcoin,anditto/bitcoin,bitcoinknots/bitcoin,qtumproject/qtum,particl/particl-core,ajtowns/bitcoin,cdecker/bitcoin,fujicoin/fujicoin,tecnovert/particl-core,mruddy/bitcoin,bitcoinsSG/bitcoin,apoelstra/bitcoin,rnicoll/bitcoin,apoelstra/bitcoin,jambolo/bitcoin,andreaskern/bitcoin,Xekyo/bitcoin,kallewoof/bitcoin,dscotese/bitcoin,AkioNak/bitcoin,mm-s/bitcoin,jambolo/bitcoin,fanquake/bitcoin,jamesob/bitcoin,fujicoin/fujicoin,andreaskern/bitcoin,EthanHeilman/bitcoin,qtumproject/qtum,domob1812/namecore,achow101/bitcoin,tecnovert/particl-core,ElementsProject/elements,ElementsProject/elements,jamesob/bitcoin,GroestlCoin/bitcoin,anditto/bitcoin,pataquets/namecoin-core,namecoin/namecoin-core,EthanHeilman/bitcoin,n1bor/bitcoin,domob1812/namecore,MeshCollider/bitcoin,prusnak/bitcoin,jlopp/statoshi,namecoin/namecoin-core,rnicoll/bitcoin,GroestlCoin/bitcoin,domob1812/bitcoin,sipsorcery/bitcoin,JeremyRubin/bitcoin,mm-s/bitcoin,Sjors/bitcoin,prusnak/bitcoin,fanquake/bitcoin,mm-s/bitcoin,GroestlCoin/GroestlCoin,ajtowns/bitcoin,namecoin/namecore,pataquets/namecoin-core,kallewoof/bitcoin,sipsorcery/bitcoin,dscotese/bitcoin,mruddy/bitcoin,qtumproject/qtum,bitcoinknots/bitcoin,MeshCollider/bitcoin,yenliangl/bitcoin,pstratem/bitcoin,cdecker/bitcoin,jamesob/bitcoin,domob1812/namecore,namecoin/namecore,bitcoinsSG/bitcoin,namecoin/namecoin-core,lateminer/bitcoin,JeremyRubin/bitcoin,tecnovert/particl-core,rnicoll/bitcoin,bitcoinknots/bitcoin,AkioNak/bitcoin,fujicoin/fujicoin,achow101/bitcoin,MarcoFalke/bitcoin,Xekyo/bitcoin,namecoin/namecore,ElementsProject/elements,sstone/bitcoin,achow101/bitcoin,bitcoin/bitcoin,prusnak/bitcoin,sipsorcery/bitcoin,andreaskern/bitcoin,apoelstra/bitcoin,MeshCollider/bitcoin,GroestlCoin/bitcoin,namecoin/namecoin-core,mruddy/bitcoin,bitcoinsSG/bitcoin,tecnovert/particl-core,jnewbery/bitcoin,cdecker/bitcoin,lateminer/bitcoin,qtumproject/qtum,MarcoFalke/bitcoin,prusnak/bitcoin,tecnovert/particl-core,anditto/bitcoin,cdecker/bitcoin,GroestlCoin/GroestlCoin,mm-s/bitcoin,n1bor/bitcoin,bitcoin/bitcoin,Xekyo/bitcoin,lateminer/bitcoin,ajtowns/bitcoin,yenliangl/bitcoin,practicalswift/bitcoin,EthanHeilman/bitcoin,jambolo/bitcoin,andreaskern/bitcoin,namecoin/namecore,rnicoll/bitcoin,fujicoin/fujicoin,AkioNak/bitcoin,pataquets/namecoin-core,apoelstra/bitcoin,MarcoFalke/bitcoin,jamesob/bitcoin,dscotese/bitcoin,domob1812/bitcoin,kallewoof/bitcoin,GroestlCoin/bitcoin,practicalswift/bitcoin,GroestlCoin/bitcoin,tecnovert/particl-core,JeremyRubin/bitcoin,apoelstra/bitcoin,Xekyo/bitcoin,MeshCollider/bitcoin,jlopp/statoshi