docs: Mention limitations of `init`
docs: Mention limitations of `init`

We now mention the limitations of `kelda init` and the `baseInfrastructure`,
so users know when to use `Machine`s directly instead.

kklin/kelda,kklin/quilt,LuiseV/quilt,kklin/kelda,kelda/kelda,kklin/kelda,kelda/kelda,LuiseV/quilt,kelda/kelda,kklin/quilt,kklin/quilt,quilt/quilt,kklin/quilt,LuiseV/quilt,quilt/quilt,quilt/quilt,NetSys/quilt,kelda/kelda,NetSys/quilt,NetSys/quilt,LuiseV/quilt,LuiseV/quilt,NetSys/quilt,quilt/quilt,kklin/kelda,NetSys/quilt