chore(workflows) : Add a README explaining our CI

.github/workflows/README.md

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+# Github Actions deployment & checks
+All of our CI checks and deployments are managed through Github actions.
+
+## Automatic checks
+Every pull request gets first checked through the jobs in `pipeline.yml`.
+
+Depending on the sub-directory you were working on, a series of linters
+and test suites will be run and verify that no assertion is broken.
+
+Of course, this does not ensure that your changes do not have an impact
+on the data or features of the product; our coverage can't be very high
+as, being a data project, we can't test our changes at scale, on real data,
+almost by definition.
+
+## Deploying the pipeline to staging
+Which is why there is a way to test your code in the `staging` area; to
+enable a deployment in staging -and thus, removing the **pipeline** code that
+would be currently deployed there- you need to choose the `deploy-to-staging`
+label in Github to immediately trigger a deployment of your branch to the
+`staging` platform.
+
+Once this label is set, any further push or change on the pull request will
+trigger a new deployment. It is expected to only have one of these labels in
+all of our PRs at one given moment, even though it is currently not enforced
+for practical reasons (it would be a little annoying)
+
+Most of the time you'll still have to manually execute DAGs or DBT steps in
+the staging environment to get to a dataset that validates your changes.
+
+## Deploying the API to staging
+The `staging` API in Scalingo will remain untouched, as it keeps following the
+`main` branch for its deployments only. If you also want to test changes in the
+API code in staging, you are welcome to trigger a manual deployment of it in
+the Scalingo dashboard or though its CLI.
+
+## Maintaining the staging data
+It is most welcome to always have the staging data the closest possible from the
+production data, so that both the Dora team and our beloved POs can do their
+jobs without having too many artifacts.
+
+It is our job to maintain the `staging` environment as stable and close to the
+production data as possible.
+
+## Deploying in production
+Merging a PR to the `main` branch does not automatically deploy its code in production.
+
+The `prod` environment follows the `release` branch, and Github only triggers a
+deployment once manually validated in its interface (at the time of writing).
+
+Thus, you have to manually rebase (fast-forward) the `release` branch to the latest `main`
+head when you're ready to deploy to prod (and upload the new reference to Github)
+
+Once the pipeline is successfully deployed to Scaleway's `prod` environment, an API
+deployment on the `data-inclusion-api-prod` project is triggered through a hook
+in Scalingo.