RHEL version support (docker#1833)

RHEL up to version 7.2 is mentioned as supported in the "follow these instructions to install CS.." section however in the "Install on Centos & RHEL section RHEL 7.3 instructions are given

Author: Brian Kaufman

cs-engine/1.13/index.md

@@ -15,7 +15,7 @@ supported version of Docker Engine.
 CS Docker Engine can be installed on the following operating systems:
 
 
-* CentOS 7.1/7.2 & RHEL 7.0/7.1/7.2 (YUM-based systems)
+* CentOS 7.1/7.2 & RHEL 7.0/7.1/7.2/7.3 (YUM-based systems)
 * Ubuntu 14.04 LTS or 16.04 LTS
 * SUSE Linux Enterprise 12