add README.md to deploy_systemd_resolved_config role

roles/deploy_systemd_resolved_config/README.md

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+# Role `deploy_systemd_resolved_config`
+
+A role for deploying a minimal configuration for [systemd-resolved](https://man.archlinux.org/man/systemd-resolved.8) or alternatively completely disabling it.
+
+!! Note
+If systemd-resolved is disabled, the configuration is instead rendered directly into `/etc/resolv.conf` to ensure a node does not accidentally lose name resolving capabilities.
+
+## Optional Arguments
+
+- `deploy_systemd_resolved_config__enable` (defaults to `true`) decides whether systemd-resolved should be enabled or disabled.
+
+- `deploy_systemd_resolved_config__mode` (defaults to `stub`) controls which compatibility mode is used for `/etc/resolv.conf` when systemd-resolved is enabled. See [man systemd-resolved(8)](https://man.archlinux.org/man/systemd-resolved.8#/ETC/RESOLV.CONF).
+
+- `deploy_systemd_resolved_config__dns` is the list of primary DNS servers that will be configured. If e.g. a specific link configures other DNS servers, they will take precedence.
+
+- `deploy_systemd_resolved_config__fallback_dns` (defaults to Quad9) is the list of fallback DNS servers. If, at runtime, none of the configured primary DNS servers are reachable, these servers will be used as fallback.
+
+## Hosts
+
+This role is included as a dependency to [base_config](../base_config/) and therefore does not need to be explicitly pulled in.
+