Improve documentation regarding plugins inherited by the starter-parent

[snicoll]

https://twitter.com/gz_k/status/693822039984095232. In particular start-class should be better documented I think. And we also need to make more clear that the repackage goal is invoked automatically.

[gzoritchak]

More than documentation, a warning in the logs would save a lot of time:
«You have defined exec.mainClass for the exec:plugin. It will not be used because spring boot ....».

[gzoritchak]

You don't consider this to be magic, maybe it's not. But it is modifying a very popular plugin making it not working... Maybe you have good reason for that but there should some warnings.

[snicoll]

We can't issue a log as you're asking. This is just a default configuration of the Maven build. Nothing forces you to use the parent if you don't want to.

If you want to run your app, you should use mvn spring-boot:run.