update readme.md with example for maven 2
update readme.md with example for maven 2

With Version 0.0.17 org.codehaus.plexus.util.Scanner was introduced in MojoUtils.java, which is a dependency of maven 3 but not maven 2. 
The turn to make the plugin maven 3 only was done in version 0.0.23.
So we need to make the difference clear for people still depending on maven 2.
eirslett/frontend-maven-plugin,stevenbower/frontend-maven-plugin,mriehema/frontend-maven-plugin,stevenbower/frontend-maven-plugin,josmardias/frontend-maven-plugin,eirslett/frontend-maven-plugin,mriehema/frontend-maven-plugin,stevenbower/frontend-maven-plugin,josmardias/frontend-maven-plugin,mriehema/frontend-maven-plugin,eirslett/frontend-maven-plugin,eirslett/frontend-maven-plugin