It is probably worth mentioning in the yocto-autobuilder-helper how buildtools-extended-tarball is integrated and also the use-cases it is intended to solve (distro buildtools too old, such as CentOS-7 or distro buildtools too new, such as trying to build an older release on OpenSUSE Tumbleweed or other distro that has rapid uptake of new versions of gcc, etc.)

Of course there are many other features in yocto-autobuilder-helper that are not documented in the somewhat sparse README, so adding this may be awkward.