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.c

@@ -21,6 +21,8 @@ void config_cb(const char *name, const char *value)
 		ctx.cfg.root_title = xstrdup(value);
 	else if (!strcmp(name, "root-desc"))
 		ctx.cfg.root_desc = xstrdup(value);
+	else if (!strcmp(name, "root-readme"))
+		ctx.cfg.root_readme = xstrdup(value);
 	else if (!strcmp(name, "css"))
 		ctx.cfg.css = xstrdup(value);
 	else if (!strcmp(name, "logo"))