docs: add development description

README.md

@@ -20,3 +20,15 @@ implementations since it handles RSA keys.
 This library does _not_ provide authentication helpers for web applications.
 If you want to use `passport js` and OAuth 2.0 introspection, head over to
 [node-passport-zitadel](https://github.com/buehler/node-passport-zitadel).
+
+### Development
+
+To enhance this library or to fix a bug, you need to do the following:
+
+1. Clone the repository
+2. Install the dependencies with `npm install`
+3. Install the submodules with `git submodule update --init --recursive`
+4. Generate the gRPC types with `npm run build:grpc`
+
+Then you can start developing with `npm run dev`. The production build can be
+made with `npm run build` and the tests run with `npm test`.