building_firmware_images.md: instructions to produce Chromebook firmware images

Move some documentation by gtucker from an Etherpad doc to
a new file and link it from README.md. The new document
(building_firmware_images.md) includes the steps to create
a Chromebook firmware image for LAVA from scratch and some examples.
Signed-off-by: Ricardo Cañuelo <ricardo.canuelo@collabora.com>

README.md

@@ -71,7 +71,7 @@ The containers are using the following local directories on the host:
 * firmware: where the firmware binaries are kept and new ones are placed
 * chroot-*: dedicated chroot directory for each device type
 
-## Chrombeook device types
+## Chromebook device types
 
 Each device type uses a different revision of the Chromium OS source tree,
 which means a different Docker image and a different Docker container with a
@@ -133,3 +133,7 @@ If everything went well, there should be a new firmware image:
 -rw-r--r-- 1 cros-build chronos 16777216 Oct 21 10:20 firmware/octopus-new.bin
 ```
 This can be accessed from the host, in the `firmware` directory.
+
+A more in-depth walkthrough of this process, including Depthcharge
+customization and the FW image generation details can be found in the
+[building_firmware_images.md document](building_firmware_images.md).