readme: explain briefly cluster expand

Expand cluster section added based on suggestion from fbalak.

The procedure described in this commit is based on ongoing specification
for cluster expansion:

Tendrl/specifications#254

That said, the expand procedure is not fully finished at the time of
writing this commit, see also:

Tendrl/commons#849

README.md

@@ -7,7 +7,8 @@ playbooks for [Tendrl](http://tendrl.org/)!
 
 ## What does it do?
 
-tendrl-ansible automates **installation of Tendrl** based description from
+tendrl-ansible automates **installation of Tendrl** and helps with **cluster
+expansion** based description from
 [Tendrl wiki](https://github.com/Tendrl/documentation/wiki). You
 should check [installation documentation
 there](https://github.com/Tendrl/documentation/wiki/Tendrl-release-latest) to
@@ -279,6 +280,43 @@ tendrl-ansible:
  on [TEN-257](https://tendrl.atlassian.net/browse/TEN-257)).
 
 
+## How do I expand cluster with tendrl-ansbile?
+
+See [Tendrl wiki](https://github.com/Tendrl/documentation/wiki) for full
+details of cluster expansion procedure. This section contains only brief
+overview of the expand operation for you to understand how tendrl-ansible
+fits into Tendrl cluster expand operation.
+
+1) First of all, you need to install operating system and Gluster on new
+ servers(s) and add them into existing cluster (aka [Gluster *Trusted Storage
+ Pool*](http://docs.gluster.org/en/latest/Administrator%20Guide/Storage%20Pools/))
+ via peer probe and add bricks on new server(s) into existing gluster
+ volume(s) based on your needs.
+
+2) When Gluster is aware of new servers (you see them in output of `gluster
+ pool list` command), you add the new servers into ansible inventory file
+ (into group `gluster-servers`) which you used during installation of
+ Tendrl.
+
+ Note that it's important to **add new servers into the same inventory file
+ as was used during installation**, because you need to ensure that you are
+ using the same set of ansible variables.
+
+3) Then, you rerun ansible playbook in the same way as done during Tendrl
+ installation:
+
+ ```
+ $ ansible-playbook -i inventory_file site.yml
+ ```
+
+ During this run, ansible should report "ok" status for
+ already existing machines, while reporting "changed" status for the new
+ machines you just added.
+
+4) Now, you should be able to see new servers in Tendrl web ui (see Tendrl
+ documentation for details).
+
+
 ## License
 
 Distributed under the terms of the [GNU LGPL, version