Re-add Maven Flatten Plugin file
Re-add Maven Flatten Plugin file

The [Maven Flatten Plugin](https://www.mojohaus.org/flatten-maven-plugin/) generates a '.flattened-pom.xml' which should usually not be committed to git. The plugin is needed e.g. for continuous integration with multi-module projects.

This had already been fixed in #221, but the change was lost again.
dvcs/gitignore