doc: add etcd to test dependencies

The tests require etcd version 3+ to be installed to work.

Author: patapenka-alexey

README.md

@@ -178,6 +178,7 @@ To run tests:
 -   [docker](https://docs.docker.com/engine/install/)
 -   [NodeJS](https://nodejs.org/en/download)
 -   [CMake](https://cmake.org/install/)
+-   [etcd 3+](https://etcd.io/docs/v3.5/install/)
 
 #### Build
 
@@ -223,6 +224,12 @@ TT_CLI_BUILD_SSL=shared mage build
 
 #### Run tests
 
+Disable `etcd` service:
+``` console
+systemctl stop etcd
+systemctl disable etcd
+```
+
 Create a virtual environment and install tests requirements:
 
 ``` console