lxd-ci is missing instructions on how to easily build an image

[simondeziel]

It'd be nice to have a section of the README in lxd-ci.git explaining how to build an image. ATM, I think the "documentation" is to look at what happens in the GitHub actions and turn that into an invocation of ./bin/build-distro. So far, I'm referring back to canonical/lxd-ci#163 (comment) as it's easier :)