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>
diff --git a/docs/manual/customize-outside-br.txt b/docs/manual/customize-outside-br.txt
index 2c3bc52..348f2b0 100644
--- a/docs/manual/customize-outside-br.txt
+++ b/docs/manual/customize-outside-br.txt
@@ -202,7 +202,9 @@
   will then be listed in the toolchain selection;
 * +provides/jpeg.in+ defines the alternative libjpeg implementations;
 * +provides/openssl.in+ defines the alternative openssl implementations;
-* +provides/skeleton.in+ defines the alternative skeleton implementations.
+* +provides/skeleton.in+ defines the alternative skeleton implementations;
+* +provides/init.in+ defines the alternative init system implementations, this
+  can be used to select a default skeleton for your init.
 
 ===== Free-form content
 
@@ -302,6 +304,13 @@
   |     |$(eval $(autotools-package))
   |     `----
   |
+  |- provides/init.in
+  |     |config BR2_INIT_MINE
+  |     |    bool "my custom init"
+  |     |    select BR2_PACKAGE_MY_INIT
+  |     |    select BR2_PACKAGE_SKELETON_INIT_MINE if BR2_ROOTFS_SKELETON_DEFAULT
+  |     `----
+  |
   |- provides/skeleton.in
   |     |config BR2_ROOTFS_SKELETON_MINE
   |     |    bool "my custom skeleton"