Prepare for 'about site' page / add 'root-readme' option to cgitrc

The new option names a file which will be included on a new page, next
to the current 'index' page.

Signed-off-by: Lars Hjemli <hjemli@gmail.com>

cgit.h

@@ -133,6 +133,7 @@ struct cgit_config {
 	char *robots;
 	char *root_title;
 	char *root_desc;
+	char *root_readme;
 	char *script_name;
 	char *virtual_root;
 	int cache_dynamic_ttl;