Visual Studio forgets code outside of the asset tree

I'm trying to write a game where lots of stuff will be outside of the main unity scripts and the Unity scripts will be referencing them. The whole thing starts in a folder that's in the root project folder.

Under MonoDevelop, in the previous version of Unity, I successfully managed to add and maintain an external library outside the assets folder for use in my code in this way in another project. However, every time I open Visual Studio with the new version of Unity, it forgets all the classes I added externally to assets.

I have no idea how to make this work. All the docs I read indicate that just by adding things once and saving them, everything should come good. But that's not happening here and I guess something must be happening under the hood, but my Visual Studio fu isn't good enough to handle it.

Any suggestions?