docs/manual: add missing external toolchain *.mk inclusion in sample

If follow through the customize-outside-br.txt with how to add external
toolchain in br-ext tree then one thing is missing - inclusion of
*.mk file with external toolchain package description.

Signed-off-by: Vadim Kochan <vadim4j@gmail.com>
Acked-by: Yann E. MORIN <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>

docs/manual/customize-outside-br.txt

@@ -246,6 +246,7 @@ illustration, of course):
   |
   |- external.mk
   |     |include $(sort $(wildcard $(BR2_EXTERNAL_BAR_42_PATH)/package/*/*.mk))
+  |     |include $(sort $(wildcard $(BR2_EXTERNAL_BAR_42_PATH)/toolchain/*/*.mk))
   |     |
   |     |flash-my-board:
   |     |    $(BR2_EXTERNAL_BAR_42_PATH)/board/my-board/flash-image \