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>