README: add short description of OE-Core

(From OE-Core rev: 5ebaabe8ed518b92515ab74963a4849410be44aa)

Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

README

@@ -20,6 +20,10 @@ The Yocto Project has extensive documentation about the system including a
 reference manual which can be found at:
     http://yoctoproject.org/community/documentation
 
-For information about OpenEmbedded see their website:
+OpenEmbedded-Core is a layer containing the core metadata for current versions
+of OpenEmbedded. It is distro-less (can build a functional image with
+DISTRO = "") and contains only emulated machine support.
+
+For information about OpenEmbedded, see the OpenEmbedded website:
     http://www.openembedded.org/