Updated help text to indicate that tag-prefix is currently the only configuration directive supported by /overrideconfig

Author: labmonkey42

src/GitVersionExe/HelpWriter.cs

@@ -26,6 +26,7 @@ path            The directory containing .git. If not defined current directory
     /l              Path to logfile.
     /showconfig     Outputs the effective GitVersion config (defaults + custom from GitVersion.yaml) in yaml format
     /overrideconfig Overrides GitVersion config values inline (semicolon-separated key value pairs e.g. /overrideconfig:tag-prefix=Foo)
+                    Currently supported config overrides: tag-prefix
 
     # AssemblyInfo updating
     /updateassemblyinfo