[PATCH v2 1/3] doc: Add imager dependencies section

[Anton Mikanovich <amikan@ilbers.de>]

Include into user_manual.md explanation of imager dependencies should
be added if using wic image type with bootloader-related plugins.

Inspired by issue #69

Signed-off-by: Anton Mikanovich <amikan@ilbers.de>
---
 doc/user_manual.md | 16 ++++++++++++++++
 1 file changed, 16 insertions(+)

diff --git a/doc/user_manual.md b/doc/user_manual.md
index 01fe701..1b00a47 100644
--- a/doc/user_manual.md
+++ b/doc/user_manual.md
@@ -586,6 +586,22 @@ Isar contains additional image type classes that can be used as reference:
  - `ubi-img`
  - `wic-img`
 
+### Imager dependencies and bootloaders
+
+External tools used for final image prepare inside image type classes should be declared as `IMAGER_INSTALL`.
+It will make Isar to install following packages into buildchroot before proceeding.
+
+If using `wic-img` image type correct bootloader dependecy should be added to target machine config based on selected wic plugin.
+
+When using legacy BIOS bootloader (amd64/i386 only) dependency on syslinux should be used:
+```
+IMAGER_INSTALL += "${SYSLINUX_BOOTLOADER_INSTALL}"
+```
+When using EFI bootloader dependency on GRUB should be added:
+```
+IMAGER_INSTALL += "${GRUB_BOOTLOADER_INSTALL}"
+```
+
 ---
 
 ## Customize and configure image
-- 
2.25.1