docs: add diagram to readme

README.md

@@ -23,3 +23,33 @@ Built with [openpassport](https://openpassport.app).
 1. On mobile app
    1. Scan QR Code shown in desktop's browser.
    2. Tap _verify_.
+
+## What is going on?
+
+![workflow diagram](./assets/dog18-diagram.svg)
+
+1. With the OpenPassport mobile app, the passport electronic data is transferred to the mobile device by near field communication.
+2. With [`@openpassport/sdk`](https://github.com/zk-passport/openpassport/tree/main/sdk), one can generate a QR code in the web client to request the generation of a specific proof (particular set of requirements).
+3. Upon scanning this QR code, a web socket connection is opened between the mobile device and the web client.
+4. The user initiates the generation of the proof on the mobile device.
+5. The mobile device transfers this proof to the web client.
+6. The web client can now perform a verification ([`@openpassport/sdk`](https://github.com/zk-passport/openpassport) < [`snarkjs`](https://github.com/iden3/snarkjs)) of this proof or transfer this proof for an authentication server to verify it.
+7. Depending on the verification result, the web client can authorize access to some restricted content.
+
+## How it was built
+
+### Reused as is
+
+- circuits from openpassport: circom/snarkjs
+- openpassport mobile app
+
+### Reused and modified
+
+- `@openpassport/sdk`
+  - I had issues with the packaging of this upstream dependency: it was mixing different targets and would lead to issues (e.g react stuff being imported server side). So in the [verification server](./server), I more or less copy pasted the original code into (see [`lib/vendor`](./server/src/lib/vendor)), cutting out the part not needed server side (react components etc...)
+  - I refactored a bit the original [`OpenPassport1StepQRcode`](./client/src/components/OpenPassportQRCode) to skip the verification client side, as I wanted to perform it server side (avoiding doing it twice), and to split it in different sub components and hooks.
+
+### Implemented from scratch
+
+- web client: react, tailwindcss
+- verification server: express