Add instructions to generate the goals documentation during site phase

In the http://maven.apache.org/guides/development/guide-plugin-documentation.html page instructions should be added to tell plugin developers they need to add the maven-plugin-plugin in the reporting section to instruct the maven-site-plugin to produce the plugin-info.html page.

I suggest to place those instructions at the end of the "Generated Documentation" section after the statement "Documentation is generated by running" adding something like "Ensure you have the maven-plugin-plugin available in the reporting section of your plugin POM" followed by the usual POM excerpt.