Add CHANGELOG.md

Author: amacneil

CHANGELOG.md

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+# Changelog :zap:
+
+# 1.0.0 (2013-06-24)
+
+`amount` is now specified as a decimal (i.e. `'10.00'` instead of `1000`
+to represent $10.00. Passing integers will throw an exception, reminding you
+to update your application code. To be clear, that means instead of this:
+
+    $gateway->purchase(array('amount' => 1000, 'currency' => 'USD'));
+
+You must now create the request like so:
+
+    $gateway->purchase(array('amount' => '10.00', 'currency' => 'USD'));
+
+This should avoid any further confusion over how to specify the amount.
+
+*   Added Mollie payment gateway
+*   Added notifyUrl and issuer fields to example app