Check status.maven.org before releasing

Author: felixbarny

CONTRIBUTING.md

@@ -227,6 +227,7 @@ See [`apm-agent-plugins/README.md`](apm-agent-plugins/README.md)
 
 If you have access to make releases, the process is as follows:
 
+1. Check if sonatype is up: https://status.maven.org
 1. Review project version. The release version will be `${project.version}` without the `-SNAPSHOT`. 
    1. In case you want to update the version, execute `mvn release:update-versions`
 1. Execute the release Jenkins job on the internal ci server. This job is same as the snapshot-build job, but it also: