no native snapd package on Centos

[noelmcloughlin]

This PR is to ensure snapd state does not run on Centos.

ID: pkg_req_pkgs
    Function: pkg.installed
      Result: False
     Comment: Error occurred installing package(s). Additional info follows:

              errors:
                  - Running scope as unit run-16643.scope.
                    Loaded plugins: fastestmirror, versionlock
                    Loading mirror speeds from cached hostfile
                     * base: linux.cc.lehigh.edu
                     * epel: mirror.sfo12.us.leaseweb.net
                     * extras: linux.cc.lehigh.edu
                     * updates: mirror.net.cen.ct.gov
                    No package snapd available.
                    Error: Nothing to do

[javierbertoli]

@noelmcloughlin can you add a line or two in the README, stating that it's not supported on CentOS?

[noelmcloughlin]

Done, thanks for your suggestion.

[javierbertoli]

Thanks @noelmcloughlin