While travelling, it would be convenient to set up a wifi hotspot from my Macbook, which would share the wifi internet with my phone and tablet (wifi <-> Macbook <-> wifi). This would make it easy to test apps I'm developing on the Macbook and also make it easier to connect devices to the internet, instead of making each of them jump through the considerable hoops of public wifi providers.
There's no ethernet involved, just a public/hotel wifi connection. I realise this isn't possible with just the built-in wifi; it needs some kind of external wifi device. My question is firstly to confirm this is possible, and secondly to ask exactly what setup (hardware and config) is required.