Added Checkout.com to Readme

Author: beinbm

README.md

@@ -110,6 +110,7 @@ Gateway | Composer Package | Maintainer
 [Buckaroo](https://github.com/thephpleague/omnipay-buckaroo) | omnipay/buckaroo | [Kayla Daniels](https://github.com/kayladnls)
 [CardGate](https://github.com/cardgate/omnipay-cardgate) | cardgate/omnipay-cardgate | [CardGate](https://github.com/cardgate)
 [CardSave](https://github.com/thephpleague/omnipay-cardsave) | omnipay/cardsave | [Kayla Daniels](https://github.com/kayladnls)
+[Checkout.com](https://github.com/fotografde/omnipay-checkoutcom) | fotografde/checkoutcom | [fotograf.de](https://github.com/fotografde)
 [Coinbase](https://github.com/thephpleague/omnipay-coinbase) | omnipay/coinbase | [Kayla Daniels](https://github.com/kayladnls)
 [Cybersource](https://github.com/dioscouri/omnipay-cybersource) | dioscouri/omnipay-cybersource | [Dioscouri Design](https://github.com/dioscouri)
 [Cybersource SOAP](https://github.com/DABSquared/omnipay-cybersource-soap) | dabsquared/omnipay-cybersource-soap | [DABSquared](https://github.com/DABSquared)