Add how-to about running the tour offline

[wilzbach]

From @wilzbach on June 13, 2016 17:1

As in the go tour, we could have a short page or subpage in the first pages that explains how to run the tour via dub.

Follow-up to #250

Copied from original issue: dlang-tour/core#260

[wilzbach]

From @stonemaster on June 14, 2016 16:30

Dependency: #137

[wilzbach]

@stonemaster afaict you need to release new git versions once you fixed something - dub will download the latest version automatically, not the latest git version.
There is also dub fetch dlang-tour --version="~master", but I am not sure whether we should recommend this command.

[wilzbach]

Just bumping this. Basically we only need to decide whether we advertise the use of --version="~master" (imho it does make sense because for now it saves us a release process and our latest, greatest content is on "master").

... and then put the commands on the welcome pages:

dub fetch dlang-tour  --version="~master"
dub run dlang-tour

[stonemaster]

I am definitely for master way because this saves us a lot of process. And basically we have a stable master approach so we should always make sure that master is the latest and greatest and always running.

[wilzbach]

This still depends on dlang-tour/core#443