Document activation step before running.

docs/installation-development.md

@@ -191,6 +191,13 @@ Use the convencience script `run.sh` to get engine started in different ways:
 
 **Run the Engine**
 
+If you have defined a virtual env during the installation step you'll ned to activate it whenever
+you gonna start the engine:
+
+```shell
+source python/bin/activate
+```
+
 This includes the Liquidsoap audio engine, but does not start the API server.
 
 ```shell