A library based on NETStandard 1.6 would look like this:
{
"version": "1.0.0",
"dependencies": {
"NETStandard.Library": "1.6.1", //nuget dependency
},
"frameworks": { //frameworks the library is build for
"netstandard1.6": {}
},
"buildOptions": {
"debugType": "portable"
}
}