Particle Cloud SDK for C# PCL Projects, tested specifically against Xamarin projects. This does work with Xamrarin.Forms as well. The SDK is a PCL based on .Net 4.6, if you have any issues installing this package into ANY C# project, try downloading the source and incorporating it directly and retargetting the solution. It could be that your framework isn't in the targetFramework for this nuget package.
$ dotnet add package ParticleXamarinSDKNo README available.