Explain how to setup a new runner for the project

.github/workflows/README.md

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+This explains how to install a cardano runner for the Hydra project.
+
+This runner is used by our smoke-tests and allows us to keep on disk
+an, as up to date as possible, cardano-node database.
+
+# prepare pre-requisites
+
+Install the following pre-requisites:
+* git
+
+For instance on Debian:
+```bash
+sudo apt install git
+```
+
+Prepare the common directory for cardano database:
+```bash
+sudo mkdir -p /srv/var/cardano
+sudo chown /srv/var/cardano "$(whoami)"
+```
+
+# Add the server as a github runner
+
+In the project settings, go to Actions/Runners and click on [New self-hostd runner](https://github.com/input-output-hk/hydra/settings/actions/runners/new) and follow the procedure.
+
+:warning: When asked, add the following label to this runner: `cardano`
+
+# Customize github runner for nix
+
+So that the jobs can find nix later, you should customize the runner settings by adding some
+variables to the `.env` file:
+
+```bash
+cat <<EOF >>$HOME/actions-runner/.env
+PATH=$HOME/.nix-profile/bin:/nix/var/nix/profiles/default/bin:/usr/local/bin:/usr/bin:/bin
+NIX_PROFILES="/nix/var/nix/profiles/default $HOME/.nix-profile"
+NIX_SSL_CERT_FILE="/etc/ssl/certs/ca-certificates.crt"
+EOF
+```
+
+# Install github runner as a systemd unit
+
+
+So that github runner runs as a daemon on the machine, [install it](https://docs.github.com/en/actions/hosting-your-own-runners/configuring-the-self-hosted-runner-application-as-a-service):
+
+
+```bash
+sudo ./svc.sh install
+```
+
+You can now start the service:
+
+```bash
+actions.runner.input-output-hk-hydra.to-remove.service
+```