Describe cli gradle task usage for composite project

[syncro]

Current composite docs section describes only arch principles that may be not enough to figure out how to build and run projects from command-line interface. The following note should be added:

Composite project has target application sub-projects that provide their names into gradle task list, so in case application project is named as "sales" the building command will looks like:

./gradlew -Pvaadin.productionMode=true :sales:bootJar