contrib/raftexample: Update README to reflect dynamic configuration changes (fixes #4018)

contrib/raftexample/README.md

@@ -65,6 +65,36 @@ goreman run start raftexample2
 curl -L http://127.0.0.1:22380/my-key
 ```
 
+### Dynamic cluster reconfiguration
+
+Nodes can be added to or removed from a running cluster using requests to the REST API.
+
+For example, suppose we have a 3-node cluster that was started with the commands:
+```sh
+raftexample --id 1 --cluster http://127.0.0.1:12379,http://127.0.0.1:22379,http://127.0.0.1:32379 --port 12380
+raftexample --id 2 --cluster http://127.0.0.1:12379,http://127.0.0.1:22379,http://127.0.0.1:32379 --port 22380
+raftexample --id 3 --cluster http://127.0.0.1:12379,http://127.0.0.1:22379,http://127.0.0.1:32379 --port 32380
+```
+
+A fourth node with ID 4 can be added by issuing a POST:
+```sh
+curl -L http://127.0.0.1:12380/4 -XPOST -d http://127.0.0.1:42379
+```
+
+Then the new node can be started as the others were, using the --join option:
+```sh
+raftexample --id 4 --cluster http://127.0.0.1:12379,http://127.0.0.1:22379,http://127.0.0.1:32379,http://127.0.0.1:42379 --port 42380 --join
+```
+
+The new node should join the cluster and be able to service key/value requests.
+
+We can remove a node using a DELETE request:
+```sh
+curl -L http://127.0.0.1:12380/3 -XDELETE
+```
+
+Node 3 should shut itself down once the cluster has processed this request.
+
 ## Design
 
 The raftexample consists of three components: a raft-backed key-value store, a REST API server, and a raft consensus server based on etcd's raft implementation.
@@ -87,4 +117,3 @@ For raftexample, this commit channel is consumed by the key-value store.
 
 ### TODO
 - Snapshot support
-- Dynamic reconfiguration