Ignore files created by maven-release-plugin

Maven Release Plugin provides a standard mechanism to release project
artifacts.

During the various phases of the execution the plugin create various
files, especially if you do a dry run that allows you to see the changes
that the plugin will perform on your pom files

mvn release:prepare -D dryRun=true

It will create

• release.properties - copy of the properties to be used
• pom.xml.releaseBackup - the pom.xml prior to any changes
• pom.xml.tag - the pom.xml as it will look when tagged
• pom.xml.next - the pom.xml as it will look for the next iteration