* `kleaf_repo_dir` was shortened to `kleaf_repo` in init_ddk script to
  keep it consistent with the flag here, so it's updated accordingly.
* This also adds a brief quickstart guide in README.md file.

Bug: 328770706
Change-Id: Iafb13701a4d02a43a0cc19f3b1ac9db5325b0a5b
Signed-off-by: Ulises Mendez Martinez <umendez@google.com>