README.md: add section about building

Separate the instructions to clone the repostory with Git LFS into a
separate section, and add a section with some minimal instructions to
get started for buildnthg Chromebook firmware images.
Signed-off-by: Guillaume Tucker <guillaume.tucker@collabora.com>

README.md

 Chromebook Firmware Utilities
 -----------------------------
 
-# Flashing
+# Installing
 
-The `servoflash.py` tool can be used to flash firmware images onto Chromebook
-devices using a Servo interface.  To use it, clone this repository with Git LFS
-enabled:
+This repository contains tools to build and flash Chromebook firmwares.  It
+uses Git LFS in order to manage binary files, so please follow these steps to
+be able to use it:
 
 ```
-sudo apt install git-lfs
+sudo apt install git-lfs  # on Debian...
 git clone https://gitlab.collabora.com/chromium/flash-tools.git
 cd flash-tools
 git lfs fetch
 git lfs checkout
 ```
 
-This includes a specific `flashrom` binary with its library dependencies copied
-from the Chromium OS SDK.
+# Flashing
+
+The [`servoflash.py`](https://gitlab.collabora.com/chromium/flash-tools/-/blob/master/servoflash.py) tool can be used to flash firmware images onto Chromebook
+devices using a Servo interface.  It uses a specific `flashrom` binary with its
+library dependencies copied from the Chromium OS SDK.  They can be found in the
+local `bin` and `lib` directories in this repository.
 
-Then to flash a firmware image, the target device needs to be in the Google
-Servo config file which is typically `/etc/google-servo.conf`.  This is where
-the device serial numbers are associated with their names.  There also needs to
-be a `servod` service running to be able to access the device.
+To flash a firmware image, the target device needs to be in the Google Servo
+config file which is typically `/etc/google-servo.conf`.  This is where the
+device serial numbers are associated with their names.  There also needs to be
+a `servod` service running to be able to access the device.
+
+Here's a sample command to flash a firmware, given all the preconditions
+mentioned above are met:
 
-Here's a sample command to flash a firmware, given all the preconditions are
-met:
 ```
 ./servoflash.py \
   --device=rk3399-gru-kevin-cbg-0 \
@@ -33,7 +38,33 @@ met:
 
 It can take a few minutes.  There should be these messages around the end,
 which can vary depending on the type of Chromebook:
+
 ```
 Erasing and writing flash chip... Verifying flash... VERIFIED.
 SUCCESS
 ```
+
+# Building
+
+The Chromebook firmware needs to be rebuilt and flashed with extra patches and
+configuration options turned on in order to enable the serial console in the
+bootloader and boot with a kernel image and ramdisk supplied over TFTP
+interactively.  The standard firmware shipped with the products is only
+configured to boot the Chrome OS image present on the device, with no serial
+console and no way to override it.  This is not suitable for automating the
+Chromebook in a test lab such as LAVA, or for doing low-level kernel
+development.
+
+Building the firmware for a Chromebook can be non-trivial.  To make things
+easier, this repository contains a Docker container and some helper scripts to
+build firmware images for some known device types.
+
+The
+[`cros-build/bootstrap.sh`](https://gitlab.collabora.com/chromium/flash-tools/-/blob/master/cros-build/bootstrap.sh)
+script can be used to create local directories, then build the Docker container
+which will be using them and start it.  It takes only one argument which is the
+device type.  For example:
+
+```
+./bootstrap.sh octopus
+```