README.md: add with flashing instructions

Add a basic README.md with some minimal instructions about how to
flash a firmware image on a Chromebook using servoflash.py.

Signed-off-by: Guillaume Tucker <guillaume.tucker@collabora.com>

README.md

+Chromebook Firmware Utilities
+-----------------------------
+
+# Flashing
+
+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:
+
+```
+sudo apt install git-lfs
+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.
+
+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.
+
+Here's a sample command to flash a firmware, given all the preconditions are
+met:
+```
+./servoflash.py \
+  --device=rk3399-gru-kevin-cbg-0 \
+  --firmware=depthcharge-rk3399-gru-kevin-20180806.dev.bin
+```
+
+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
+```