Add Twinfield OAuth2 provider

Author: mvdpanne

docs/providers/thirdparty.md

@@ -81,6 +81,7 @@ Gateway | Composer Package | Maintainer
 [SuperJob](https://packagist.org/packages/alexmasterov/oauth2-superjob) | alexmasterov/oauth2-superjob | [Alex Masterov](https://github.com/AlexMasterov)
 [ThirtySevenSignals](https://github.com/nilesuan/oauth2-thirtysevensignals) | nilesuan/oauth2-thirtysevensignals | [Nile Suan](https://github.com/nilesuan)
 [Trakt.tv](https://github.com/Bogstag/oauth2-trakt) | bogstag/oauth2-trakt | [Krister Bogstag](https://github.com/Bogstag/)
+[Twinfield](https://github.com/php-twinfield/twinfield) | php-twinfield/twinfield | [Mollie B.V.](https://github.com/mollie)
 [Twitch.tv](https://github.com/tpavlek/oauth2-twitch) | depotwarehouse/oauth2-twitch | [Troy Pavlek](https://github.com/tpavlek)
 [Uber](https://github.com/stevenmaguire/oauth2-uber) | stevenmaguire/oauth2-uber | [Steven Maguire](https://github.com/stevenmaguire)
 [Unsplash](https://github.com/hughbertd/oauth2-unsplash) | hughbertd/oauth2-unsplash | [Hugh Downer](https://github.com/hughbertd)