Add 'Invoice protocol'

Invoice-protocol.md

@@ -0,0 +1,61 @@
+# Invoice protocol
+## Rules
+1. If a payment was made, but the funds were not sufficient, the funds will be sent back.
+2. If a payment was made, but it was not in time, it will be sent back.
+3. If a payment was made, the expiration timer will be paused.
+## Creation
+### 1. Submit a invoice request
+The first step is to tell LibrePay that you want to create a new invoice by submitting all necessary details.
+For example:
+
+```json
+POST /invoice/
+{
+    "successUrl": "http://localhost/complete",
+    "cancelUrl": "http://localhost/cancel",
+    "currency": "usd",
+    "cart": [
+        {
+            "price": 0.1,
+            "name": "Raspberry Pi 4 - 8GB RAM",
+            "image": "https://www.avxperten.dk/billeder/raspberry-pi-4-model-b-2gb-usb-c-side.jpg",
+            "quantity": 1
+        }
+    ]
+}
+```
+With that, LibrePay knows where to redirct after finishing or cancelling of the payment and how much everything will cost.
+
+LibrePay will **respond** with all currently activated crypto currencies and there corresponding price:
+```json
+Response 200 OK
+{
+    "methods": [
+        { "coin:": "BTC", "price": 0.00037000 },
+        { "coin": "XMR", "price": 0.0606816 },
+        { "coin": "ETH", "price": 0.0137751 }
+    ],
+    "selector": "YjcTCvruxPwdiNMkfmPuKkXEfPpOvahreyVVrlqqzDjqRgWwybsWTurkXWowOZZurSujSVozXRvKxxVqixFldifMZKFlJRjgUAJyoMOVrxsvxiBPwhtaeMuugfthoXer",
+    "expireDate": 1609162829
+}
+```
+Since the price for each crypto currency has been set, the user will receive a fixed time in which the payment has to be made.
+
+### 2. Choose a payment method
+Now you know what crypto currencies are supported and how much you'll have to pay. All you have to do now is to **choose one of them.**
+```json
+POST /invoice/YjcTCvruxPwdiNMk[...]/setmethod
+{
+    "method": "BTC"
+}
+```
+LibrePay acknowledged this choice and will now await payments on a specific addresss.
+```json
+Response 200 OK
+{
+    "receiveAddress": "1QFrbboisCYgeBdowGAuCTK3r2YnZuApYd"
+}
+```
+You can also get this address by calling `GET /invoice/YjcTCvruxPwdiNMk[...]`
+
+##