Make the need to clone the repository a bit more explicit.

[fungiboletus]

I tried by just downloading the docker-compose.yml file and it obviously failed.

[sscaling]

Relates to #407

[sscaling]

Essentially I'm a bit torn by this, it's an outstanding task, and it does fix one particular issue. However, if you look at the related #407 ticket you can see that the ways to connect to Kafka via the console consumer/producer have changed over the versions. So part of me thinks that the actual fix is to provide specific example docker-compose (sans the build directive) for download and use in tutorials.

With that said, probably no harm in merging this.