[doc] when and how to define plugins in a pom

some simple rules to document

1) define lifecycle per packaging, according to default behaviour
2) if a project needs to add a mojo, use <execution/> + <phase/> (if needed)
3) if a project needs to remove a mojo, make mojo configurable such that
it can be skipped via POM configuration
4) if there is a pattern in common for adjusting lifecycle for several
projects, define a new packaging