How to add files to the build?

Question

Xamarin

I'm using Xamarin.Forms (PCL) and I'm trying to add a resource (1.txt) to the project so that this file is available in the compiled version, but it's not there. I managed to google only adding images to assets, but I need to add a plain text file and open it at runtime.
What is the correct way to add resource files to the project and what will be their address during program execution?