Add required sections to pom.xml for deployment

We will release this beast via Sonatype OSS... therefore, we need to
change the pom.xml a little bit. With these changes, we can deploy
snapshots using

	mvn -Psonatype-oss-release deploy

and releases using

	mvn -Psonatype-oss-release release:clean
	mvn -Psonatype-oss-release release:prepare
	mvn -Psonatype-oss-release release:perform

followed by logging into https://oss.sonatype.org/index.html and
closing and releasing the uploaded version.

For details, see

	https://docs.sonatype.org/display/Repository/Central+Sync+Requirements
	https://docs.sonatype.org/display/Repository/How+To+Generate+PGP+Signatures+With+Maven
	https://maven.apache.org/guides/mini/guide-central-repository-upload.html
	https://docs.sonatype.org/display/Repository/Sonatype+OSS+Maven+Repository+Usage+Guide

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>