Sync usage guide with Maven standard directory layout

According to http://maven.apache.org/guides/introduction/introduction-to-the-standard-directory-layout.html and http://docs.codehaus.org/display/MAVENUSER/The+Standard+Directory+Layout, assembly descriptors should be stored under "src/main/assembly". The usage guide currenlty suggests to use "src/assembly" which misleads the reader to break with the standard directory layout.