README: Clarify where to run the generate_git script from

Just running the script from the current directory doesn't give a clue:

    Cant find series file at /series

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
1 file changed