Adding Groovy build plugin in parent pom.xml to allow for importng proje...

...cts in Eclipse

Also adding a paragraph to mention that the Groovy tooling for Eclipse should be installed before importing the projects
in the workspace.

See http://groovy.codehaus.org/Groovy-Eclipse+compiler+plugin+for+Maven#Groovy-EclipsecompilerpluginforMaven-Usethegroovy-eclipse-compilermojoforconfiguringsourcefolders for more explanation on why the build plugin was added. Without this plugin configured in the parent pom.xml, Eclipse would not use the src/main/java, src/test/java, etc. folders as source folders for the projects, resulting in many build errors in the workspace.