Added note about firewall on TKL Hub

Just added a brief note about firewall adjustments when testing via the Hub. 

Although to many (most?) this would be obvious, it tripped me up recently and i wasted a fair bit of time trying to work out why Apache wasn't working at all...

There may be other places where it could be mentioned but following a quick look this seemed like the most logical place to put it (although perhaps it should go in tkldev / docs / installation.rst where the Hub is mentioned there?)