ovmf-prebuilt: Update readme

ovmf-prebuilt/README.md

@@ -1,4 +1,33 @@
 # ovmf-prebuilt
 
-This package will contain a library for downloading prebuilts. It is not
-yet implemented.
+[![Crates.io](https://img.shields.io/crates/v/ovmf-prebuilt)][crates.io]
+[![Docs.rs](https://docs.rs/ovmf-prebuilt/badge.svg)][docs.rs]
+
+This library downloads and caches prebuilt [OVMF] binaries. See
+[docs.rs/ovmf-prebuilt][docs.rs] for a usage example.
+
+## License
+
+This packaged is under either of
+
+- Apache License, Version 2.0 ([LICENSE-APACHE](LICENSE-APACHE) or
+  http://www.apache.org/licenses/LICENSE-2.0)
+- MIT license ([LICENSE-MIT](LICENSE-MIT) or http://opensource.org/licenses/MIT)
+
+at your option.
+
+Unless you explicitly state otherwise, any contribution intentionally
+submitted for inclusion in the work by you, as defined in the Apache-2.0
+license, shall be dual licensed as above, without any additional terms
+or conditions.
+
+Note that the downloaded prebuilts are under a different license. OVMF
+is part of the [tianocore/edk2] project. See the [`License.txt`][edk2
+license] and the [OVMF wiki] for licensing information.
+
+[OVMF wiki]: https://github.com/tianocore/tianocore.github.io/wiki/OVMF
+[OVMF]: https://github.com/tianocore/edk2/tree/master/OvmfPkg#readme
+[crates.io]: https://crates.io/crates/ovmf-prebuilt
+[docs.rs]: https://docs.rs/ovmf-prebuilt
+[edk2 license]: https://github.com/tianocore/edk2/blob/master/License.txt
+[tianocore/edk2]: https://github.com/tianocore/edk2