stagit.1: clarify README and LICENSE link creation

thanks Solene Rapenne for the patch! (slightly modified)

stagit.1

@@ -70,6 +70,9 @@ owner of repository
 primary clone url of the repository, for example: git://git.2f30.org/stagit
 .El
 .Pp
+When the files README or LICENSE exist in HEAD, a direct link in the menu is
+made.
+.Pp
 For changing the style of the page you can use the following files:
 .Bl -tag -width Ds
 .It logo.png