NuGet.config

<?xml version="1.0" encoding="utf-8"?>
<configuration>
  <packageSources>
    <!--To inherit the global NuGet package sources remove the <clear/> line below -->
    <clear />
    <add key="aspnetcirelease" value="https://www.myget.org/F/aspnetcirelease/api/v3/index.json" />
    <add key="dotnet-core" value="https://www.myget.org/F/dotnet-core/api/v3/index.json" />
    <add key="api.nuget.org" value="https://api.nuget.org/v3/index.json" />
  </packageSources>
</configuration>

Program.cs

using System;

namespace MyLib
{
    public class Libbie
    {
        public static string MyLib()
        {
            return "Hello world";
        }
    }
}

project.json

{
  "version": "1.0.0-*",
  "dependencies": {
	"NETStandard.Library": "1.5.0-rc2-24022"
  },
  "frameworks": {
    "netstandard1.2": {}
  }
}