Updated usage section to talk about pros/cons of importing vs inheriting

Updated the README to mention the pros and cons of both importing and inheriting. 

JBoss projects that already inherit from the JBoss parent POM may find it much better to inherit because the project POM can see, use and override the properties defined in the BOM, including those properties used to specify the versions of artifacts.

Both importing an inheriting are valid ways to use the BOM.

UPDATE: The section about importing was taken from https://github.com/jboss-integration/jboss-integration-platform-bom/pull/11, albeit with a few refinements/additions. This PR also adds the section about inheriting. So https://github.com/jboss-integration/jboss-integration-platform-bom/pull/11 can be merged before this, or it can be replaced with this.