sdk: Improve the README

This patch adds some documentation on how to regenerate the SDK to the
README file.

Change-Id: I341c475485cdfd963190e4d04ebbea5520608927
Signed-off-by: Juan Hernandez <juan.hernandez@redhat.com>
(cherry picked from commit 354d93b7200825cfb166d9105d4dcc668623faa2)