On Windows, add a start menu link to the flag reference

distrib/ghc.iss

@@ -21,6 +21,7 @@ Source: "ghc-@VERSION@\README.txt"; DestDir: "{app}"; Flags: isreadme
 Name: "{group}\@VERSION@\GHCi"; Filename: "{app}\bin\ghci.exe"
 Name: "{group}\@VERSION@\GHC Documentation"; Filename: "{app}\doc\index.html"
 Name: "{group}\@VERSION@\GHC Library Documentation"; Filename: "{app}\doc\libraries\index.html"
+Name: "{group}\@VERSION@\GHC Flag Reference"; Filename: "{app}\doc\users_guide\flag-reference.html"
 Name: "{group}\@VERSION@\GHC Readme"; Filename: "{app}\README.txt"
 
 [Registry]