Replace the placeholder README with the full public-facing README, and
switch licensing to BSD 3-Clause per aav's decision on ANW-28: add a single
LICENSE file, drop the dual LICENSE-MIT / LICENSE-APACHE, and update the
Cargo.toml license field to match. README/LICENSE content authored by kaa,
landed verbatim.
Assumed: dropping the old MIT/Apache files and updating Cargo.toml
license = BSD-3-Clause is the consistent completion of aav's BSD decision
(v0.0.0, nothing released relied on the old grant); flag if the dual
license was meant to stay.
