sepolicy: Add an introduction to the README.

The README jumped directly into using the BOARD_SEPOLICY_*
variables for device-specific policy; add a short introduction
describing what external/sepolicy contains and noting where to put
device-specific policy.

Change-Id: I3c800df93d70074384da993a689a5a0771ecb314
Signed-off-by: Stephen Smalley <sds@tycho.nsa.gov>

README

+This directory contains the core Android SELinux policy configuration.
+It defines the domains and types for the AOSP services and apps common to
+all devices.  Device-specific policy should be placed under a
+separate device/<vendor>/<board>/sepolicy subdirectory and linked
+into the policy build as described below.
+
 Policy Generation:
 
 Additional, per device, policy files can be added into the