Updated README.md

Bumped Java and Gradle versions.
Notified users about the `null` part in the jar name.
Clarified a bit about what constitutes a valid issue.

@pahimar, would you mind putting an [embeddable build status](https://wiki.jenkins-ci.org/display/JENKINS/Embeddable+Build+Status+Plugin) in the README somwhere?

And also, if you think the link to "pahimar's Twitter" should only catch "Twitter", or "Twitter" should be lowercase, let me know ;).