add a note about building "install" first so other targets don't fail

If you clone and then try to "mvn test" it will fail with:

[INFO] ------------------------------------------------------------------------
[ERROR] BUILD ERROR
[INFO] ------------------------------------------------------------------------
[INFO] Error unpacking file: /home/tobyc/try/openmrs-module-basicmodule/api/target/classes to: /home/tobyc/try/openmrs-module-basicmodule/omod/target/classes
org.codehaus.plexus.archiver.ArchiverException: The source must not be a directory.

Building the "install" target fixes this.