Add SELinux to README and test pillars (/linux/system/selinux.sls).

README.rst

         cpu:
           governor: performance
 
+SELinux
+~~~~~~~
+
+Set SELinux mode on System:
+
+.. code-block:: yaml
+
+    linux:
+      system:
+        selinux: permissive
 
 CGROUPS
 ~~~~~~~

tests/pillar/system.sls

     domain: ci.local
     environment: prd
     purge_repos: true
+    selinux: permissive
     directory:
       /tmp/test:
         makedirs: true