Commons RDF release process

1. Update documentation (RELEASE-NOTES.md, README.md, version numbers in src/site/)
2. Clean build: mvn clean install
3. RAT checking: mvn apache-rat:check
4. Prepare the release: mvn release:prepare -DreleaseVersion=0.4.0 -DdevelopmentVersion=0.5.0-SNAPSHOT -DautoVersionSubmodules=true
5. Perform the release: mvn release:perform -Prelease
6. Close the staging repository at https://repository.apache.org/#stagingRepositories
7. Push the code: git push and tag git push --tags
8. Cast the [VOTE] mail to dev@commons

Notice that the maven-release-plugin is configured to use the local git repository as well as not push changes to origin. Therefore the process can be reverted (e.g., git reset HEAD~1) at any time before the sixth step.

Acknowledgements to the Marmotta's release process that heavily inspired this one.