This directory was missing from the chapter that presents the
Source Directory structure. I have added a brief explanation
of it based on Paul Eggleton's information.
(From yocto-docs rev: 5c3d392c64f386a3a29922629387c199db2b7ad1)
Signed-off-by: Scott Rifenbark <scott.m.rifenbark@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>