simplest solution to prevent that warning is:
<fileSets>
<fileSet>
<directory>src/main/resources</directory>
<outputDirectory/>
</fileSet>
</fileSets>
or an other solution is:
<fileSets>
<fileSet>
<directory>src/main/resources</directory>
<outputDirectory>./</outputDirectory>
</fileSet>
</fileSets>
and it shows that something should be fixed.