git:  mirror setup guide

Signed-off-by: Gokulnath Avanashilingam <Gokulnath.Avanashilingam@in.bosch.com>
git: mirror setup guide reference link update

Signed-off-by: Gokulnath Avanashilingam <Gokulnath.Avanashilingam@in.bosch.com>