rtk_flame: Add documentation for building and use

Add instructions for building the rtk_flame project and how to update
the output binary into the servod docker container.

BUG=none
TEST=View in gitiles

Change-Id: I62c5e89a1c39af198768229bd4347227305c4371
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/ec/+/7833001
Reviewed-by: caveh jalali <caveh@chromium.org>
Commit-Queue: Keith Short <keithshort@chromium.org>
Tested-by: Keith Short <keithshort@chromium.org>