Project	Path	Errors	Warnings	Messages
Documentation	Source\Documentation.shfbproj	1	0	0
ConveyorSubsystem.Documentation	Source\ConveyorSubsystem.Documentation.shfbproj	1	0	0
PISubsystem.Documentation	Source\PISubsystem.Documentation.shfbproj	1	0	0
SSubsystem.Documentation	Source\SSubsystem.Documentation.shfbproj	1	0	0
Subsystem.Documentation	Source\Subsystem.Documentation.shfbproj	1	0	0

So, I started to dig about it. 

After some research I found that shfbproj stands for Sandcastle Hefp File Builder project.

In order to force Visual Studio to work with this file, you'll need Sandcastle guided installer, that can be downloaded from here.

Unpacke and execute exe, and you'll see screen like this:

Take note that last version allows to install package also for Visual Studio 2017 as well.

After those steps, all files with shfbproj will become openable by Visual Studio 2015. 

And in opened solution you'll see icons like this:

and you'll be able to work with them.