[make] Add dependencies helper message.
[make] Add dependencies helper message.

Dependencies target was not documented in the `make help`, this commit add
a help message for the target.

Signed-off-by: Eloy Coto <2a8acc28452926ceac4d9db555411743254cf5f9@gmail.com>

3scale/docker-gateway,3scale/apicast,3scale/apicast,3scale/apicast,3scale/apicast,3scale/docker-gateway