README: clarify how to install requirements for unit testing and how to run unit tests

Author: em92

README.md

@@ -100,11 +100,17 @@ and thus not have the client download them automatically.
 
 ## Develop
 
-To run unittests, use following commands:
+To install requirements for unit testing:
 
 ```
-git submodule update
+git submodule init
+git submodule update --recursive
 python3 -m pip install mockito
+```
+
+To run unit tests, use following commands:
+
+```
 python3 -m unittest tests
 ```