[MACR-3] Maven site should have an example of adding Main Class

**[Gustavo Orair](https://issues.apache.org/jira/secure/ViewProfile.jspa?name=orair)** opened **[MACR-3](https://issues.apache.org/jira/browse/MACR-3?redirect=false)** and commented

Maven ACR plugin should confirm if the resulting Manifest contain a setting for Main-Class, because The Java EE spec requires that app clients specify the main class (http://www.java.net/forum/topic/glassfish/glassfish/problems-using-java-web-start-application-client-libraries-not-found).

If Manifest does not contain Main-Class a warning should be emitted.

Also, It would be worth if there were working examples specifying Main-Class added to the site http://maven.apache.org/plugins/maven-acr-plugin/.

Presently, users tend to create application clients without Main-Class and may get strange errors while trying to launch the application client. One example of these errors, may be accessed in:
http://www.java.net/forum/topic/glassfish/glassfish/problems-using-java-web-start-application-client-libraries-not-found

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**Affects:** 1.0

Review the Maven ACR plugin's manifest generation and the existing plugin site examples first. Confirm how a missing Main-Class is handled, then add the requested warning and a working site example showing how to configure Main-Class; done means both behaviors are documented and verifiable.

Written by the indexing model from the issue text.