What is the correct way to add netstandard(.net core) support to the library(nuget package)?

Question

.NET

There is a c# library provided by nuget package with source on github, for example https://github.com/graffen/NLog.Targets.Syslog , but it doesn't support .net core, what is the right way to add .net core support?
1) need to add .sln?
2)need to add a new project?
3) Add a dependency on which version of netstandard? (1.0, 1.3, 1.6)
4) fix builds?
5) fix the configs (which ones)?

Answer 1

My github commit , I added new sln and new .net core dll, removed everything except project.json in the project.
To copy data from the main project, use:

"buildOptions": {
     "compile": {
       "include": [ "../NLog.Targets.Syslog/**/*.cs" ]
     },
     "warningsAsErrors": false,
     "outputName": "NLog.Targets.Syslog",
     "xmlDoc": true
   },

put in code

#if NET45
#if NETSTANDARD1_6

Nuget settings are correct in the .nuspec settings commit , I didn’t do the settings correctly to the end, I don’t know how to correctly indicate that the package will be pre-release =)