README.md: improve the instructions for using Docker
There are some details missing at the Docker procedures.
Rewrite the entire chapter, being a clearer about how the
Docker support works, and pointing that the instructions
may require some distro-specific tuning, asking the user to
seek for documentation at the Internet about how to install
docker, if needed.
Signed-off-by: Mauro Carvalho Chehab <email@example.com>