Add docker build instructions to README

[EmilePapillon]

Since building the container image seems to require specifying build arguments GITREPO and BRANCH, it would be useful to some users to have a line explaining this in the README.

[lgrosz]

Poky repo: git://git.yoctoproject.org/poky
Branch: rocko or some other version

docker build -t crops/toaster --build-arg "GITREPO=git://git.yoctoproject.org/poky" --build-arg "BRANCH=rocko" .