README.md: Add instructions on configuring a venv

Following instruction for the manual deployment in README.md, it seems that one is not able to use the system's global mat2 (i.e. installed with apt isntall mat2). Instead, one has to use a virtual environment. This MR introduces the instructions to do so, as well as an additional line in config/uwsgi.config to use the virtual environment.

config/uwsgi.config

@@ -16,4 +16,5 @@ die-on-term = true
 socket = /run/uwsgi/uwsgi.sock
 chmod-socket = 774
 plugins = python3
+virtualenv = /var/www/mat2-web/venv