Update dev doc: ensure json files exist in configured build_json_dir directory
Update dev doc: ensure json files exist in configured build_json_dir directory

* CLOUDBLD-162

Signed-off-by: Chenxiong Qi <7a7314576eb0408cef5766c0268b5ff02ad3cfb6@redhat.com>

projectatomic/osbs-client,projectatomic/osbs-client