plugin should support 'help'

[hgschmie]

mvn com.facebook.presto:presto-maven-plugin:0.4:help
[INFO] Scanning for projects...
[INFO] ------------------------------------------------------------------------
[INFO] BUILD FAILURE
[INFO] ------------------------------------------------------------------------
[INFO] Total time: 0.489 s
[INFO] Finished at: 2018-02-13T14:30:34-10:00
[INFO] Final Memory: 13M/309M
[INFO] ------------------------------------------------------------------------
[ERROR] Could not find goal 'help' in plugin com.facebook.presto:presto-maven-plugin:0.4 among available goals generate-service-descriptor, check-spi-dependencies -> [Help 1]
[ERROR]
[ERROR] To see the full stack trace of the errors, re-run Maven with the -e switch.
[ERROR] Re-run Maven using the -X switch to enable full debug logging.
[ERROR]
[ERROR] For more information about the errors and possible solutions, please read the following articles:
[ERROR] [Help 1] http://cwiki.apache.org/confluence/display/MAVEN/MojoNotFoundException

adding the HelpMojo would offer basic online help.