Update README.md to add the dotnet-core feed
Update README.md to add the dotnet-core feed

ibrmohammed/openiddict-core,vyfster/openiddict-core,openiddict/openiddict-core,joshcomley/openiddict-core,openiddict/openiddict-core,PinpointTownes/openiddict-core,PinpointTownes/core,PinpointTownes/core,nklap/core,openiddict/openiddict-core,belsoft/openiddict-core,openiddict/core,ibrmohammed/openiddict-core,openiddict/openiddict-core,vyfster/openiddict-core,PinpointTownes/openiddict-core,ilmax/core,belsoft/openiddict-core,openiddict/core,ilmax/core,nklap/core,joshcomley/openiddict-core,openiddict/openiddict-core