README states BB requirement `~> 0.13`, but mix.exs pins `~> 0.16`

[jimsynz]

README.md (Requirements section) says:

• BB framework (~> 0.13)

but mix.exs actually pins the dependency at:

{:bb, bb_dep("~> 0.16")}

The two disagree. The README should match the real mix.exs pin (and per the workspace convention satellites are being bumped toward ~> 0.20 as they're touched, so this may want bumping at the same time).

Found while adding usage-rules.md (#61 / PR #62); not fixed there to keep that PR docs-only.