RN: Fix script name

Improve some text to describe the script usage.

Signed-off-by: Daiane Angolini <daiane.angolini@freescale.com>
gitignore: Add scripts/doc-data.pckl

This file is a temporary file created after the
script execution.

Signed-off-by: Daiane Angolini <daiane.angolini@freescale.com>
scripts: Point user to release-notes/README on usage

Signed-off-by: Daiane Angolini <daiane.angolini@freescale.com>