Update README.md
Add reference for the recently added CentOS 8.
@@ -41,6 +41,7 @@ SLES 12 | rpm | 2.7 | systemd
SLES 15 | rpm | 3.6 | systemd
CentOS 6 | rpm | 2.6 | upstart
CentOS 7 | rpm | 2.7 | systemd
+CentOS 8 | rpm | 3.6 | systemd
RHEL 6 | rpm | 2.6 | upstart
RHEL 7 | rpm | 2.7 | systemd
RHEL 8 | rpm | 3.6 | systemd
