[Log4j] Release in README.md

[Mikael Ståldal]

We (should) have a link to the latest release in README.md in Git. 
However, that was not updated since 2.8. I have updated it to 2.9.0 now.

We should make sure to update it for every release.
Do we have a release checklist? On that we should add to create a Git 
tag, and update README.md.