When trying to support ADFS in your solutions developed in Visual Studio 2012 you will probably run into the issue that adding a STS reference to the project is missing from the context menu in the IDE.
At first I thought it would be automatically added to the IDE by installing the Windows Identity Foundation SDK on the machine, but this was not the case.
It turns out that the way you add this to Visual Studio 2012 is by navigating to the menu-item Tools and then clicking on Extensions and Updates...
In this window you then need to click on the Online tab in the left navigation panel, and then type in identity in the top right search field.
This will list quite a few items in the main results panel. Find the item titled "Identity and Access Tool" and install it. Once installed, you will be prompted to restart your Visual Studio instance.
When you have you restarted Visual Studio, you can now bring up the context menu for the project you wish to add the STS Reference for, and click the item named Identity and Access... - note that it is no longer called Add STS Reference in the context menu any longer.