Docs: Noting deprecation of  BOARD_SEPOLICY_UNION

Bug: 20068706
Change-Id: I8fce0f03af944bc0bd8b89c0245354151f33a71a
diff --git a/src/devices/tech/security/selinux/implement.jd b/src/devices/tech/security/selinux/implement.jd
index aa89303..0655e1d 100644
--- a/src/devices/tech/security/selinux/implement.jd
+++ b/src/devices/tech/security/selinux/implement.jd
@@ -140,6 +140,11 @@
         sepolicy.te
 </pre>
 
+<p class="note"><strong>Note:</strong> As of the M release,
+BOARD_SEPOLICY_UNION is no longer required as all policy files found within any
+directory included in the BOARD_SEPOLICY_DIRS variable are joined with the
+base policy automatically.</p>
+
 <p>After rebuilding your device, it is enabled with SELinux. You can now either
 customize your SELinux policies to accommodate your own additions to the
 Android operating system as described in <a