I've created a new project in Visual Studio 2015 with ASP.NET 5. I've created a web application, now I want to add Sitecore to the project but I'm not able to find any documentation on this.
So my question is where I can find a guide how to implement Sitecore 8 and C#? Is this even possible?
UPDATE. My fault I understood your question incorrectly :)
As far as I know Sitecore doesn't support ASP.NET 5.0 yet. I checked the ASP.NET 5 road map (ASP.NET Core 1.0 is the new name) and it looks like the first version will be this year (https://github.com/aspnet/Home/wiki/Roadmap). So I suggest to not use Sitecore with ASP.NET 5 until Sitecore will officially support it.
Some other guys asked same question on community: https://community.sitecore.net/developers/f/8/t/440
https://community.sitecore.net/developers/f/5/t/736
I also added Sitecore Compatibility Table
https://kb.sitecore.net/articles/087164 from the comments.