Update README.md

Add PRIVATE_KEY_ENCRYPTED key to the example config array, since the key is almost always encrypted.

Author: panda-madness

README.md

@@ -13,6 +13,7 @@ $client = new \Epay\Client(array(
     'MERCHANT_NAME'           => 'merchant name',
     'PRIVATE_KEY_FN'          => 'private key filename',
     'PRIVATE_KEY_PASS'        => 'private key password',
+    'PRIVATE_KEY_ENCRYPTED'   => 1,
     'XML_TEMPLATE_FN'         => 'xml template filename',
     'XML_TEMPLATE_CONFIRM_FN' => 'xml confirmation template filename',
     'PUBLIC_KEY_FN'           => 'public key filename',