fix python docs and added venv (#640)

websockets/chat/README.md

@@ -32,7 +32,9 @@ Use two tabs to set up a proper conversation.
 ## Python Client using aiohttp
 
 Client connects to server. Reads input from stdin and sends to server.
-Fetch the needed python libraries `pip3 install --requirements requirements.txt`
+Create a venv environment `python3 -m venv venv`
+Launch venv environment `source ./venv/bin/activate`
+Fetch the needed python libraries `pip3 install -r requirements.txt`
 Then start client as `./client.py`