Document multi-module configuration with suppression file

I had the same issue as reported in #711. I tried @jeremylong's suggestion of following the approach recommended by maven-checkstyle-plugin. It worked for `mvn verify` but failed on `mvn -B dependency-check:update-only`

```
[ERROR] No plugin found for prefix 'dependency-check' in the current project and in the plugin groups [org.apache.maven.plugins, org.codehaus.mojo] available from the repositories [local (/path/to/user/dir/.m2/repository), central (https://repo1.maven.org/maven2)] -> [Help 1]
```

It'd be awesome if the proper approach for handling multi-module configuration with a suppression file was documented on the usage page.