docs: add intro, development and contribution notes to readme

Author: Felix Peters

README.md

@@ -5,6 +5,12 @@ This repository contains the keycloak-spi-trusted-device Keycloak plugin.
 The goal of this plugin is to enable users who log in using multi-factor authentication to trust
 their device, so they do not have to provide a second factor when using that device.
 
+The feature is mentioned as a native capability in Keycloak [here](https://github.com/keycloak/keycloak/issues/8742),
+but it has not been implemented yet, and it's uncertain if it ever will be.
+Therefore, we are currently implementing it as an extension.
+
+The functionality is illustrated in the following diagram. Details about the individual components are provided below.
+
 ![Authentication flow](doc/authentication-flow.png)
 
 ## Usage
@@ -38,3 +44,24 @@ During login the user will be prompted if they want to trust the device:
 They can then manage their trusted devices on the account console:
 
 ![](doc/account-console.png)
+
+## Development
+
+To set up a local environment, first build the extension using Maven. Then, build a Docker image and start the Docker Compose stack.
+
+```bash
+mvn package -DskipTests
+docker-compose up --build -d
+```
+
+A helper script including this command can be found [here](run-dev.sh)
+
+## Testing
+
+Testing is not implemented yet.
+
+## Contributions Welcome
+
+We welcome contributions from the community! If you would like to contribute, please submit a merge request.
+If you have any questions or encounter any issues, feel free to open an issue.
+We appreciate your help in improving this project.