[IMP] doc: add a warning about POS & HTTPS

The POSBox do not work over HTTPS, the pos must be in HTTP.
See #3156 for details

doc/setup/deploy.rst

@@ -231,6 +231,12 @@ just about any SSL termination proxy, but requires the following setup:
 * your SSL termination proxy should also automatically redirect non-secure
 connections to the secure port
 
+.. warning::
+
+  In case you are using the Point of Sale module in combinaison with a `POSBox`_,
+  you must disable the HTTPS configuration for the route ``/pos/web`` to avoid
+  mix-content errors.
+
 Configuration sample
 --------------------
 
@@ -422,3 +428,4 @@ which will generate a 32 characters pseudorandom printable string.
 .. _use an SSH tunnel:
     http://www.postgresql.org/docs/9.3/static/ssh-tunnels.html
 .. _WSGI: http://wsgi.readthedocs.org/
+.. _POSBox: https://www.odoo.com/page/point-of-sale-hardware#part_2