Add buildNumber.properties

The popular build plugin org.codehaus.mojo:buildnumber-maven-plugin stores the build number it managers in a file named buildNumber.properties.

Maven.gitignore

@@ -5,3 +5,4 @@ pom.xml.versionsBackup
 pom.xml.next
 release.properties
 dependency-reduced-pom.xml
+buildNumber.properties