docs/manual: add section about br2-external init

Signed-off-by: Brandon Maier <brandon.maier@rockwellcollins.com>
[yann.morin.1998@free.fr:
  - move before skeletons
  - select the custom 'mine' skeleton, not the sysv one
]
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>
1 file changed