Add Adyen API/CSE/HPP (thephpleague#542)

* Add Adyen API/CSE/HPP

There are a number of older Omnipay Adyen drivers on packagist, none maintained for a number of years, none using the newer features, and none submitted to this list. I'm happy to look at merging any features in that they have and I may have missed.

* Added list newline

Author: judgej

README.md

@@ -100,6 +100,7 @@ Gateway | 2.x | 3.x | Composer Package | Maintainer
 [2c2p](https://github.com/dilab/omnipay-2c2p) | ✓ | ✓ | dilab/omnipay-2c2p | [Xu Ding](https://github.com/dilab)
 [2Checkout](https://github.com/thephpleague/omnipay-2checkout) | ✓ | - | omnipay/2checkout | [Omnipay](https://github.com/thephpleague/omnipay)
 [2Checkout Improved](https://github.com/collizo4sky/omnipay-2checkout) | ✓ | - | collizo4sky/omnipay-2checkout | [Agbonghama Collins](https://github.com/collizo4sky)
+[Adyen](https://github.com/academe/omnipay-adyen) | - | ✓ | academe/omnipay-adyen | [Jason Judge](https://github.com/judgej)
 [Agms](https://github.com/agmscode/omnipay-agms) | ✓ | - | agmscode/omnipay-agms | [Maanas Royy](https://github.com/maanas)
 [Alipay(Global)](https://github.com/lokielse/omnipay-global-alipay) | ✓ | ✓ | lokielse/omnipay-global-alipay | [Loki Else](https://github.com/lokielse)
 [Alipay](https://github.com/lokielse/omnipay-alipay) | ✓ | ✓ | lokielse/omnipay-alipay | [Loki Else](https://github.com/lokielse)