docs: create security policy
docs: create security policy

Github has started supporting SECURITY.md to contain a project's
security policy. Right now, the only place to find this project's
security contact is on bitcoincore.org. Adding this information to the
repository makes it easier to find as SECURITY.md becomes a standard.

This is copied almost exactly from https://bitcoincore.org/en/contact/
and based on conversations with Ethan Heilman.

ElementsProject/elements,lateminer/bitcoin,mitchellcash/bitcoin,mruddy/bitcoin,AkioNak/bitcoin,wellenreiter01/Feathercoin,EthanHeilman/bitcoin,Sjors/bitcoin,litecoin-project/litecoin,domob1812/bitcoin,DigitalPandacoin/pandacoin,litecoin-project/litecoin,apoelstra/bitcoin,pataquets/namecoin-core,peercoin/peercoin,MeshCollider/bitcoin,fanquake/bitcoin,CryptArc/bitcoin,jtimon/bitcoin,namecoin/namecoin-core,kallewoof/bitcoin,GroestlCoin/bitcoin,AkioNak/bitcoin,jonasschnelli/bitcoin,FeatherCoin/Feathercoin,nikkitan/bitcoin,rnicoll/dogecoin,practicalswift/bitcoin,mitchellcash/bitcoin,anditto/bitcoin,domob1812/namecore,tjps/bitcoin,namecoin/namecoin-core,pataquets/namecoin-core,instagibbs/bitcoin,Xekyo/bitcoin,domob1812/namecore,fanquake/bitcoin,rnicoll/bitcoin,MarcoFalke/bitcoin,FeatherCoin/Feathercoin,MeshCollider/bitcoin,fanquake/bitcoin,bitcoinknots/bitcoin,practicalswift/bitcoin,tecnovert/particl-core,jambolo/bitcoin,particl/particl-core,bitcoin/bitcoin,litecoin-project/litecoin,mm-s/bitcoin,lateminer/bitcoin,ahmedbodi/vertcoin,particl/particl-core,fujicoin/fujicoin,ahmedbodi/vertcoin,ajtowns/bitcoin,lateminer/bitcoin,jamesob/bitcoin,tecnovert/particl-core,particl/particl-core,FeatherCoin/Feathercoin,jamesob/bitcoin,dscotese/bitcoin,JeremyRubin/bitcoin,rnicoll/dogecoin,sstone/bitcoin,EthanHeilman/bitcoin,AkioNak/bitcoin,lateminer/bitcoin,fanquake/bitcoin,sipsorcery/bitcoin,mm-s/bitcoin,nikkitan/bitcoin,wellenreiter01/Feathercoin,pstratem/bitcoin,Xekyo/bitcoin,litecoin-project/litecoin,jamesob/bitcoin,kallewoof/bitcoin,GroestlCoin/bitcoin,qtumproject/qtum,ajtowns/bitcoin,pataquets/namecoin-core,apoelstra/bitcoin,dscotese/bitcoin,gjhiggins/vcoincore,sipsorcery/bitcoin,sipsorcery/bitcoin,namecoin/namecore,rnicoll/bitcoin,gjhiggins/vcoincore,FeatherCoin/Feathercoin,ahmedbodi/vertcoin,Xekyo/bitcoin,GroestlCoin/bitcoin,namecoin/namecore,peercoin/peercoin,Xekyo/bitcoin,GroestlCoin/bitcoin,dscotese/bitcoin,mruddy/bitcoin,MarcoFalke/bitcoin,CryptArc/bitcoin,monacoinproject/monacoin,jamesob/bitcoin,bitcoinknots/bitcoin,anditto/bitcoin,GroestlCoin/GroestlCoin,monacoinproject/monacoin,FeatherCoin/Feathercoin,yenliangl/bitcoin,prusnak/bitcoin,pstratem/bitcoin,pstratem/bitcoin,anditto/bitcoin,anditto/bitcoin,yenliangl/bitcoin,tjps/bitcoin,jtimon/bitcoin,vertcoin/vertcoin,MeshCollider/bitcoin,mitchellcash/bitcoin,DigitalPandacoin/pandacoin,bitcoinknots/bitcoin,MarcoFalke/bitcoin,tjps/bitcoin,qtumproject/qtum,jambolo/bitcoin,jnewbery/bitcoin,n1bor/bitcoin,alecalve/bitcoin,particl/particl-core,jnewbery/bitcoin,mitchellcash/bitcoin,namecoin/namecoin-core,instagibbs/bitcoin,bitcoin/bitcoin,n1bor/bitcoin,cdecker/bitcoin,alecalve/bitcoin,mm-s/bitcoin,alecalve/bitcoin,mruddy/bitcoin,CryptArc/bitcoin,alecalve/bitcoin,nikkitan/bitcoin,midnightmagic/bitcoin,jambolo/bitcoin,mruddy/bitcoin,jamesob/bitcoin,pstratem/bitcoin,Sjors/bitcoin,vertcoin/vertcoin,sipsorcery/bitcoin,gjhiggins/vcoincore,ajtowns/bitcoin,JeremyRubin/bitcoin,CryptArc/bitcoin,anditto/bitcoin,qtumproject/qtum,gjhiggins/vcoincore,rnicoll/dogecoin,nikkitan/bitcoin,jamesob/bitcoin,jlopp/statoshi,mm-s/bitcoin,jtimon/bitcoin,domob1812/namecore,apoelstra/bitcoin,jnewbery/bitcoin,bitcoinsSG/bitcoin,GroestlCoin/GroestlCoin,nikkitan/bitcoin,DigitalPandacoin/pandacoin,apoelstra/bitcoin,alecalve/bitcoin,namecoin/namecore,wellenreiter01/Feathercoin,yenliangl/bitcoin,monacoinproject/monacoin,andreaskern/bitcoin,qtumproject/qtum,achow101/bitcoin,bitcoinsSG/bitcoin,achow101/bitcoin,peercoin/peercoin,achow101/bitcoin,pstratem/bitcoin,ahmedbodi/vertcoin,tecnovert/particl-core,prusnak/bitcoin,Xekyo/bitcoin,bitcoin/bitcoin,fujicoin/fujicoin,fanquake/bitcoin,litecoin-project/litecoin,midnightmagic/bitcoin,monacoinproject/monacoin,DigitalPandacoin/pandacoin,rnicoll/bitcoin,qtumproject/qtum,bitcoinknots/bitcoin,tecnovert/particl-core,ElementsProject/elements,kallewoof/bitcoin,practicalswift/bitcoin,EthanHeilman/bitcoin,MarcoFalke/bitcoin,mitchellcash/bitcoin,midnightmagic/bitcoin,pataquets/namecoin-core,jlopp/statoshi,monacoinproject/monacoin,vertcoin/vertcoin,jambolo/bitcoin,ahmedbodi/vertcoin,instagibbs/bitcoin,jonasschnelli/bitcoin,sstone/bitcoin,fanquake/bitcoin,domob1812/namecore,JeremyRubin/bitcoin,domob1812/bitcoin,domob1812/namecore,vertcoin/vertcoin,bitcoinsSG/bitcoin,achow101/bitcoin,jlopp/statoshi,tecnovert/particl-core,FeatherCoin/Feathercoin,fujicoin/fujicoin,prusnak/bitcoin,MeshCollider/bitcoin,ajtowns/bitcoin,anditto/bitcoin,bitcoinknots/bitcoin,pataquets/namecoin-core,domob1812/bitcoin,achow101/bitcoin,Sjors/bitcoin,mitchellcash/bitcoin,Sjors/bitcoin,ajtowns/bitcoin,qtumproject/qtum,fujicoin/fujicoin,pstratem/bitcoin,particl/particl-core,gjhiggins/vcoincore,mm-s/bitcoin,AkioNak/bitcoin,namecoin/namecore,tjps/bitcoin,rnicoll/bitcoin,jonasschnelli/bitcoin,instagibbs/bitcoin,achow101/bitcoin,yenliangl/bitcoin,EthanHeilman/bitcoin,prusnak/bitcoin,DigitalPandacoin/pandacoin,sstone/bitcoin,domob1812/bitcoin,JeremyRubin/bitcoin,Sjors/bitcoin,jtimon/bitcoin,AkioNak/bitcoin,tjps/bitcoin,qtumproject/qtum,bitcoin/bitcoin,jlopp/statoshi,cdecker/bitcoin,fujicoin/fujicoin,monacoinproject/monacoin,mm-s/bitcoin,n1bor/bitcoin,jnewbery/bitcoin,ElementsProject/elements,namecoin/namecoin-core,domob1812/bitcoin,midnightmagic/bitcoin,EthanHeilman/bitcoin,jlopp/statoshi,lateminer/bitcoin,midnightmagic/bitcoin,prusnak/bitcoin,jnewbery/bitcoin,peercoin/peercoin,andreaskern/bitcoin,tecnovert/particl-core,namecoin/namecoin-core,instagibbs/bitcoin,namecoin/namecore,namecoin/namecore,bitcoinsSG/bitcoin,AkioNak/bitcoin,JeremyRubin/bitcoin,rnicoll/bitcoin,CryptArc/bitcoin,sstone/bitcoin,jonasschnelli/bitcoin,bitcoinsSG/bitcoin,MeshCollider/bitcoin,nikkitan/bitcoin,practicalswift/bitcoin,ahmedbodi/vertcoin,particl/particl-core,andreaskern/bitcoin,alecalve/bitcoin,fujicoin/fujicoin,practicalswift/bitcoin,rnicoll/dogecoin,cdecker/bitcoin,GroestlCoin/bitcoin,bitcoin/bitcoin,GroestlCoin/GroestlCoin,tjps/bitcoin,rnicoll/dogecoin,rnicoll/bitcoin,jambolo/bitcoin,litecoin-project/litecoin,Xekyo/bitcoin,jtimon/bitcoin,domob1812/namecore,pataquets/namecoin-core,vertcoin/vertcoin,dscotese/bitcoin,prusnak/bitcoin,kallewoof/bitcoin,instagibbs/bitcoin,CryptArc/bitcoin,bitcoinsSG/bitcoin,sstone/bitcoin,n1bor/bitcoin,ElementsProject/elements,kallewoof/bitcoin,dscotese/bitcoin,apoelstra/bitcoin,JeremyRubin/bitcoin,jtimon/bitcoin,vertcoin/vertcoin,ajtowns/bitcoin,sstone/bitcoin,EthanHeilman/bitcoin,jonasschnelli/bitcoin,n1bor/bitcoin,GroestlCoin/GroestlCoin,MeshCollider/bitcoin,peercoin/peercoin,practicalswift/bitcoin,yenliangl/bitcoin,mruddy/bitcoin,ElementsProject/elements,apoelstra/bitcoin,andreaskern/bitcoin,namecoin/namecoin-core,GroestlCoin/bitcoin,cdecker/bitcoin,ElementsProject/elements,midnightmagic/bitcoin,jambolo/bitcoin,peercoin/peercoin,sipsorcery/bitcoin,MarcoFalke/bitcoin,cdecker/bitcoin,sipsorcery/bitcoin,domob1812/bitcoin,mruddy/bitcoin,n1bor/bitcoin,gjhiggins/vcoincore,wellenreiter01/Feathercoin,kallewoof/bitcoin,dscotese/bitcoin,yenliangl/bitcoin,andreaskern/bitcoin,wellenreiter01/Feathercoin,lateminer/bitcoin,GroestlCoin/GroestlCoin,GroestlCoin/GroestlCoin,cdecker/bitcoin,andreaskern/bitcoin,bitcoin/bitcoin,DigitalPandacoin/pandacoin,jlopp/statoshi,MarcoFalke/bitcoin,wellenreiter01/Feathercoin