manual/user guide/customization: rework 'step-by-step instructions'

This patch reworks the section 'Step-by-step instructions for storing
configuration' as follows:
- rename into 'Quick guide to storing your project-specific customizations'
  and hence make it serve as a summary to the chapter
- change the introduction to make this clear
- update paths to line-up with section 'recommended directory structure'
- recommend BR2_GLOBAL_PATCH_DIR, as is done in the section on adding
  patches.
- mention how to add packages in a project-specific directory (to be
  expanded in another patch)
- minor rewording
- rename file into customize-quick-guide.txt

Signed-off-by: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>