gitweb/README: Document $base_url

Signed-off-by: Jakub Narebski <jnareb@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

gitweb/README

@@ -165,6 +165,12 @@ not include variables usually directly set during build):
    Full URL and absolute URL of gitweb script;
    in earlier versions of gitweb you might have need to set those
    variables, now there should be no need to do it.
+ * $base_url
+   Base URL for relative URLs in pages generated by gitweb,
+   (e.g. $logo, $favicon, @stylesheets if they are relative URLs),
+   needed and used only for URLs with nonempty PATH_INFO via
+   <base href="$base_url>.  Usually gitweb sets its value correctly,
+   and there is no need to set this variable, e.g. to $my_uri or "/".
  * $home_link
    Target of the home link on top of all pages (the first part of view
    "breadcrumbs").  By default set to absolute URI of a page ($my_uri).