Update readme

plugins/drivers/readme.txt

@@ -0,0 +1,11 @@
+Enable drivers in this directory like this:
+
+<?php
+function adminer_object() {
+	include "./plugins/drivers/simpledb.php"; // the driver is enabled just by including
+	return new Adminer; // or return AdminerPlugin if you want to use other plugins
+}
+
+// include original Adminer
+include "./adminer.php";
+?>