begin integration of payment gateways

src/server/backend/plugins/payments/README.md

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+# Creating new payment provider integrations
+All payment gateway integration plugins have to be in their own folders. This folder has to contain a file called [Payment gateway name as camelCase word (starting in small letter)]Routes.js, a file called plugin.json and any number of other files that it needs. 
+
+You will also need to add documentation for the user to set up the payment gateway. Read on below to find out how.
+
+## Setting up the routes.js file
+In the routes.js file you should have at least the following code:
+
+```
+    module.exports = ( app ) => {
+
+    }
+```
+
+Take some inspiration of the stripe or adyen setup as these are officially supported by the system and have been developed by the original creator.
+
+The express.js routes it has to expose are the following:
+
+- /payments/prepare (POST) (returns object: { 'link': [Link to the payment gateway payment], 'id': [Purchase ID to subscribe to status updating] })
+- /payments/getStatus (SSE) (sends updates to the UI once subscribed (like payment success) using the id sent by /payments/prepare)
+
+It can contain any number of (not interfering) routes. Please always use the /payments/ route as a base to avoid running into problems.
+
+
+## The plugin.json file
+The plugin.json file should look as follows: