Cannot use Facepunch Steamworks library

I downloaded this release of Facepunch Steamworks:

Then I took Unity folder from the release archive, renamed it and copied to Assets/Plugins. I created a new script

using UnityEngine;
using Steamworks;

namespace Astro
{
    public class SteamworksManager : MonoBehaviour
    {
    } 
}

But I get compilation error saying that the "Steamworks namespace cannot be found".
I tried reloading Unity and Visual Studio. Also I tried using Facepunch.Steamworks; It also not working.

How to make this work?

I had the same issue when using Unity on Linux. I think I fixed it after opening the Properties of Facepunch.Steamworks.Posix.dll and changing the OS in Platform settings from OSX to Linux.

Just for additional clarification I add a link to the appropriate issue on GitHub and an image of the solution: