diff --git a/watcher/CONTRIBUTING.md b/watcher/CONTRIBUTING.md
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+# Contributing
+
+In order to test this component, you need:
+- a test Zimfarm instance with a username + password
+- a test S3 bucket  (compliance must NOT be activated on this bucket)
+- credentials to access this bucket (keyId and secretAccessKey suggested below)
+- Docker
+
+Rebuild the Docker image:
+
+```
+docker build -t local-zf-watcher .
+```
+
+Export the secret `S3_URL` as environment variable. Note that the S3 URL starts with `https`.
+
+
+On Bash/Zsh shells (replace `<your_s3_host_name>`, `<your_key_id>`, `<your_secret_access_key>` and `<your_bucket>` with proper values):
+
+```
+ export S3_URL="https://<your_s3_host_name>/?keyId=<your_key_id>&secretAccessKey=<your_secret_access_key>&bucketName=<your_bucket>"
+```
+
+Run a test (here my zimfarm is running in docker on a container `backend` in network `zimfarm_default`, adapt command to your local setup):
+```
+docker run -it --rm -e ZIMFARM_API_URL=http://backend:8000/v1 -e S3_URL=$S3_URL --network zimfarm_default local-zf-watcher watcher --zimfarm-username admin --zimfarm-password admin --only tezos.stackexchange.com --runonce
+```
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