Add a documentation on how to extend a single-node keeper

docs/en/guides/sre/keeper/index.md

@@ -1150,7 +1150,18 @@ server.id2 = ...
   Priority of 0 means server will never be a leader.
 
 You can use `reconfig` command to add new servers, remove existing ones, and change existing servers'
-priorities, here are examples (using `kazoo`):
+priorities, here are examples (using `clickhouse-keeper-client`):
+
+```bash
+# Add two new servers
+reconfig add "server.5=localhost:123,server.6=localhost:234;learner"
+# Remove two other servers
+reconfig remove "3,4"
+# Change existing server priority to 8
+reconfig add "server.5=localhost:5123;participant;8"
+```
+
+And here are examples for `kazoo`:
 
 ```python
 # Add two new servers, remove two other servers
@@ -1192,6 +1203,20 @@ There are some caveats in Keeper reconfiguration implementation:
 - `reconfig` command may fail for various reasons. You can check cluster's state and see whether the update
   was applied.
 
+## Converting a single-node keeper into a cluster
+
+Sometimes it's necessary to extend experimental keeper node into a cluster. Here's a scheme of how to do it step-by-step for 3 nodes cluster:
+
+- **IMPORTANT**: new nodes must be added in batches less than the current quorum, otherwise they will elect a leader among them. In this example one by one.
+- The existing keeper node must have `keeper_server.enable_reconfiguration` configuration parameter turned on.
+- Start a second node with the full new configuration of keeper cluster.
+- After it's started, add it to the node 1 using [reconfig](#reconfiguration).
+- Now, start a third node and add it using [reconfig](#reconfiguration).
+- Update the `clickhouse-server` configuration by adding new keeper node there and restart it to apply the changes.
+- Update the raft configuration of the node 1 and, optionally, restart it.
+
+To get confident with the process, here's a [sandbox repository](https://github.com/ClickHouse/keeper-extend-cluster).
+
 ## Unsupported Features
 
 While ClickHouse Keeper aims to be fully compatible with ZooKeeper, there are some features that are currently not implemented (although development is ongoing):