Markdown Documentation extension is available in Extension Registry. Markdown Documentation editor allows you to edit documentation of an element in support of Markdown syntax coloring and previewing. You can install via Extension Manager (Tools > Extension Manager).
Edit mode
Preview mode
StarUML 
It would be nice if you could dock Markdown Documentation panel in other positions than the center bottom. A “snap to dock” gizmo similar to Visual Studio’s would b perfect. Nice job!!