To do this you will need to save your workflow as an Analytic App, if you haven't already.
In the Interface Designer (View->Interface Designer), under the Properties setting - the wrench icon, there are two fields - "Output Message" and "No Output Files Message". The first will display if there is output that will appear on the Gallery, and the second will display if there isn't. So you could easily use the "No Output Files Message" to hard-code in some information.
If you are looking for something more configurable, you can use Report Text and the Render Tool to create a temporary formatted document which will save with the execution when it completes. Let me know if you are looking for this level of configuration and I will put together a quick example.