Clarify master config yaml format

Setting up a demo buildbot-travis instance on a dedicated machine, then accessing the web interface from another machine, results in the error / warning

```
Warning: c['buildbotURL'] is misconfigured to
http://localhost:8010/
Should be:
http://buildbot:8010/
```
grepping through all the config files to identify the faulty setting only yields `master.cfg.sample`. The real `master.cfg` file uses the `TravisConfigurator` to read the config in from `cfg.yml`, but that neither contains a `buildbotURL` nor do I find any documentation for where to add it.