footer: link back to cgit home page

The footer has always been overrideable using the footer= in cgitrc, so
this won't anger anybody who cares about their footer.

ui-shared.c

@@ -740,7 +740,7 @@ void cgit_print_docend()
 	if (ctx.cfg.footer)
 		html_include(ctx.cfg.footer);
 	else {
-		htmlf("<div class='footer'>generated  by cgit %s at ",
+		htmlf("<div class='footer'>generated by <a href='http://git.zx2c4.com/cgit/about/'>cgit %s</a> at ",
 			cgit_version);
 		cgit_print_date(time(NULL), FMT_LONGDATE, ctx.cfg.local_time);
 		html("</div>\n");