[FIX] doc: mention psycogreen dependency for gevent worker

doc/setup/deploy.rst

@@ -126,6 +126,10 @@ Instead you must have a proxy redirecting requests whose URL starts with
 ``/longpolling/`` to the longpolling port. Other request should be proxied to
 the :option:`normal HTTP port <odoo.py --xmlrpc-port>`
 
+.. warning:: The livechat worker requires the ``psycogreen`` Python module,
+             which is not always included with all installation packages.
+             It can be manually installed with ``pip install psycogreen``.
+
 Odoo as a WSGI Application
 ==========================