manual/user guide/customization: add section on project-specific packages

This patch adds a new section to chapter 'Project-specific customization' to
describe how to add project-specific packages from a project-specific
directory. The principle was already described in the presentation 'Using
Buildroot for real projects' but was never documented in official Buildroot
documentation.

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