Add README instructions to test the flasher tool (#674)

Co-authored-by: Luca Rinaldi <lucarin@protonmail.com>

Author: MatteoPologruto

arduino-flasher-cli/README.md

@@ -0,0 +1,43 @@
+# Arduino Flasher CLI
+
+A tool to download and flash Debian images on the board.
+
+## Build and test it locally
+
+Build it with `task arduino-flasher-cli:build` and run `task flash:staging` to download and flash the latest image from staging.
+
+Alternatively, these are the steps to test it locally:
+
+1. download a debian image release (for example from [here](https:/downloads.oniudra.cc/debian-im/Stable/20250902-166/arduino-unoq-debian-image-20250902-166.tar.xz), then create the following structure under `arduino-flasher-cli/`:
+
+```
+arduino-flasher-cli/public/
+└── debian-im
+    └── Stable
+        ├── 20250902-166
+        │   └── arduino-unoq-debian-image-20250902-166.tar.xz
+        └── info.json
+```
+
+2. Populate `info.json`:
+
+```
+{
+  "latest": {
+    "version": "20250902-166",
+    "url": "http://127.0.0.1:3001/debian-im/Stable/20250902-166/arduino-unoq-debian-image-20250902-166.tar.xz",
+    "md5sum": "ad0aac0a9b18982e9dce0dd99808a5e5"
+  }
+}
+```
+
+3. `task debian:release-server:start`
+4. `UPDATE_URL=http://127.0.0.1:3001 ./build/arduino-flasher-cli flash latest`
+
+## Flash from staging
+
+To download and flash the latest image from staging, the tool needs cloudflare credentials. This is the command that should be used:
+
+```sh
+CF_ACCESS_CLIENT_ID="$(aws ssm get-parameter --profile arduino-staging --with-decryption --name /devops/downloads/cloudflare_access_client_id --query Parameter.Value --output text)" CF_ACCESS_CLIENT_SECRET="$(aws ssm get-parameter --profile arduino-staging --with-decryption --name /devops/downloads/cloudflare_access_client_secret --query Parameter.Value --output text)" ./arduino-flasher-cli flash latest
+```