Document <args> in JLink examples

[loiclefevre]

In JLink examples page, it would be interesting to provide examples about passing args and so documenting (here for Maven) the tag.

This would help to understand that to add a new arg, you need this syntax:

...
</mainJar>
<args>
    <arg>--no-header-files</arg> <!-- default -->
    <arg>--no-man-pages</arg>  <!-- default -->
    <arg>--compress=2</arg>      <!-- default -->
    <arg>--strip-debug</arg>       <!-- default -->
    <arg>--ignore-signing-information</arg>
</args>
<jdk>
...

Thanks