manual/user guide/customization: add section with recommended dir structure

Although the user is free to choose where to put project-specific files, the
presentation 'Using Buildroot for real projects' [1] first described a
certain recommendation.
This patch briefly describes this recommendation in the form of a directory
structure overview, the details will be added in subsequent patches.

Since the section on BR2_EXTERNAL is logically connected to the directory
structure, this section is moved right after it.

[1] http://elinux.org/images/2/2a/Using-buildroot-real-project.pdf



Signed-off-by: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>