Document details of configuring Gradle to produce valid poms

If a Gradle build is using the Spring Boot Gradle plugin's support for
declaring dependencies without versions then they will be unable to
publish Maven artifacts from the build as the Gradle-generated pom
will fail to validate. This is because Gradle doesn't apply the
Boot-provided dependency versions to the dependencies in the generated
pom.

This can be addressed by configuring Gradle to generate a pom that
either imports spring-boot-dependencies into its dependency management
or that uses spring-boot-starter-parent as its parent. This commit
updates the documentation to document the need for this configuration
and to provide examples of how to do so.

Closes gh-1806