Add a small note that mvn "package" must happen before "test"

Add to the building-with-maven.md:

Requirement: build packages before running tests
Tests must be run AFTER the "package" target has already been executed. The following is an example of a correct (build, test) sequence:
mvn -Pyarn -Phadoop-2.3 -DskipTests -Phive clean package
mvn -Pyarn -Phadoop-2.3 -Phive test

BTW Reynold Xin requested this tiny doc improvement.