This is not a new feature, but something not all.Net devs are aware of, you can share files from one project to another by linking the file in directly. Let me demonstrate, create two projects in a solution, and create a file with whatever you want shared. Add this file to just one of the projects. On the other project right click and select Add => existing item. Find the file, and click on the dropdown arrow and select add as link. Thats it. Really.
Whenever you add a change to the file it will be updated, no matter which project you are in. It is just referenced in, not copied as when you simply select Add.