docs: add instructions to add nodes (#2040)

Author: Tanguille

README.md

@@ -289,6 +289,36 @@ task talos:upgrade-k8s
 # e.g. task talos:upgrade-k8s
 ```
 
+### ➕ Adding a node to your cluster
+
+At some point you might want to expand your cluster to run more workloads and/or improve the reliability of your cluster. Keep in mind it is recommended to have an **odd number** of control plane nodes for quorum reasons.
+
+You don't need to re-bootstrap the cluster to add new nodes. Follow these steps:
+
+1. **Prepare the new node**: Review the [Stage 1: Machine Preparation](#stage-1-machine-preparation) section and boot your new node into maintenance mode.
+
+2. **Get the node information**: While the node is in maintenance mode, retrieve the disk and MAC address information needed for configuration:
+
+   ```sh
+   talosctl get disks -n <ip> --insecure
+   talosctl get links -n <ip> --insecure
+   ```
+
+3. **Update the configuration**: Read the documentation for [talhelper](https://budimanjojo.github.io/talhelper/latest/) and extend the `talconfig.yaml` file manually with the new node information (including the disk and MAC address from step 2).
+
+4. **Generate and apply the configuration**:
+
+   ```sh
+   # Render your talosconfig based on the talconfig.yaml file
+   task talos:generate-config
+
+   # Apply the configuration to the node
+   task talos:apply-node IP=?
+   # e.g. task talos:apply-node IP=10.10.10.10
+   ```
+
+The node should join the cluster automatically and workloads will be scheduled once they report as ready.
+
 ## 🤖 Renovate
 
 [Renovate](https://www.mend.io/renovate) is a tool that automates dependency management. It is designed to scan your repository around the clock and open PRs for out-of-date dependencies it finds. Common dependencies it can discover are Helm charts, container images, GitHub Actions and more! In most cases merging a PR will cause Flux to apply the update to your cluster.