readme.md in spree_mollie_gateway-1.0.4

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 This gateway seamlessly integrates with the <a href="https://guides.spreecommerce.org/api/">Spree API</a>.
 
 ## Endpoints
 
+- <a href="checkout.md">Integrating with the Spree API</a>
 - <a href="methods.md">Get Mollie payment methods</a>
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