add readme.md

Author: lucarin91

README.md

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+# Arduino App CLI
+
+`arduino-app-cli` is a command line tool and a service running on Arduino UNO Q boards, that:
+
+- manages and runs Arduino Apps on the board (both Linux and microcontroller parts)
+- provides multiple APIs to perform actions and fetch data, used by the front-end (ArduinoAppsLab)
+- auto-updates itself and other components
+
+## Environment Variables
+
+The following environment variables are used to configure `arduino-app-cli`:
+
+### Application Directories
+
+- **`ARDUINO_APP_CLI__APPS_DIR`** Path to the directory where Arduino Apps created by the user are stored.\
+  **Default:** `/home/arduino/ArduinoApps`
+
+- **`ARDUINO_APP_CLI__DATA_DIR`** Path to the directory where internal data is stored.\
+  **Default:** `/home/arduino/.local/share/arduino-app-cli`\
+  This folder contains:
+  - **`examples/`** default example Apps (_e.g._ `/home/arduino/.local/share/arduino-app-cli/examples`)
+  - **`assets/`** contains a subfolder for each asset version (_e.g._ `/home/arduino/.local/share/arduino-app-cli/assets/0.4.5`)
+    - Each asset folder includes:
+      - `bricks-list.yaml`
+      - `models-list.yaml`
+  - **other data** such as `properties.msgpack` containing variable values
+
+- **`ARDUINO_APP_BRICKS__CUSTOM_MODEL_DIR`** Path to the directory where custom models are stored.\
+  **Default:** `$HOME/.arduino-bricks/ei-models`\
+  (_e.g._ `/home/arduino/.arduino-bricks/ei-models`)
+
+---
+
+### Execution Settings
+
+- **`ARDUINO_APP_CLI__ALLOW_ROOT`** Allow running `arduino-app-cli` as root.\
+  **Default:** `false` **Not recommended to set to true.**
+
+---
+
+### External Services
+
+- **`LIBRARIES_API_URL`** URL of the external service used to search libraries.\
+  **Default:** `https://api2.arduino.cc/libraries/v1/libraries`
+
+---
+
+### Docker Settings
+
+- **`DOCKER_REGISTRY_BASE`** Docker registry used to pull images.\
+  **Default:** `ghcr.io/arduino/`
+
+- **`DOCKER_PYTHON_BASE_IMAGE`** Tag of the Docker image for the Python runner.\
+  **Default:** `app-bricks/python-apps-base:<RUNNER_VERSION>`
+
+### App folder and persistent data
+
+When running an app, persistent files will be saved in the `data` folder inside the app folder; other supporting files, including the Python venv are saved in the `.cache` folder inside the app folder.
+
+### Docker images registry
+
+Arduino Apps bricks might required a docker image, in that case the orchestrator will pull those from the registry configured with the `DOCKER_REGISTRY_BASE` environment variable. By default this points to an Arduino GitHub Container Registry (ghcr.io/arduino).
+
+The only image that needs to be referenced directly is the base Python image (`DOCKER_PYTHON_BASE_IMAGE`), all other containers can be downloaded automatically by the orchestrator depending on the bricks specified as dependencies in the app.yml file.