Documentation: Expand on installing under Docker

[andybeak]

Expand on docker example to make it easier to persist configuration changes.

The current example is not a good example of 'installing' onto Docker because it is not at all suitable for production use.

There is no clear documentation elsewhere that helps users find this. Adding it here helps point users in the right direction.

See issue #451

[vegasbrianc]

I would be happy to expand on this a bit as well. I am building an API Umbrella Swarm stack which I will document and include.

[science695]

@vegasbrianc I would also be interested in knowledge on clustering API Umbrella

[ldealmei]

@vegasbrianc same, if there are any advances towards a docker stack, I'd be very much interested!