chroot instructions are missing note to manually create bootstrap.cfg

[lrvick]

Took me a bit to work this out, so probably worth a note somewhere.

I may take this myself once I finally get the chroot working at all.

[Googulator]

It should be auto-generated, but there is currently a bug that may break that. It should be fixed by PR #354.

[Googulator]

@lrvick or is this about the manual instructions for bootstrapping without rootfs.py?